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Undertale / Tropes A to G

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You can view the main article here.
Tropes H-P here.
Tropes Q-Z here.

Note: "No Mercy" and "Genocide" are two names for the same, officially unnamed, route of the game.

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    0- 9 
  • 11th-Hour Superpower: During the final boss fight in the pacifist run, if you run out of HP, your determination stops you from dying and you revive with full HP. The second phase starts off hopeless, with you unable to do anything. Even saving the game is impossible, but then you realize you can save something else. Your ACT command then turns into SAVE, with a new icon and flashing colors, which allows you to reach inside Asriel and "save" the souls he's absorbed and eventually Asriel himself.
  • 100% Completion:
    • Deconstructed. If you want to see everything the game has to offer, you'll have to play through the game several times and take different actions, which is par for the course for most games with multiple endings; however, both Undertale and its characters remember your previous runs...
    • Certain NPCs will notice that you've played the game before. And will "point out" the illogicalities of 100% Completion in the context of the world, to put it lightly.
    • This is outright discussed on the No Mercy run during the segment in New Home. Flowey points out that he tried helping everyone, but eventually that got boring, so he tried killing everyone. He did everything he could differently in every way he could think of until he exhausted every possible outcome and began to see the people of the underground as predictable, scripted actors repeating the same dialogue over and over, so he mostly sat around waiting for something to happen. This makes him a pretty clear in-universe stand-in for the type of player who plays a game to death just to squeeze every ounce of gameplay and dialogue out of it, no matter how callous or cruel the actions they take are. Eventually, even Sans calls you out for doing this not because it's fun or challenging, but because you can – taking the No Mercy route removes most NPCs, changes the fun music to slow, ominous chords, removes all the minigames, and makes all minor battles stunningly easy; in short, it takes away everything about the game that makes people want to play it and makes it actively demoralizing, so Sans knows that the only reason anyone would have gotten this far is because they want the 100%.
    • The game does provide a pacifist completionist goal, however; in the true credits, monster names will be either yellow or white depending on if you've fulfilled a specific condition to make them especially happy or took an easy way out. To make every monster's name yellow, one has to encounter every monster — with some exceptions, such as So Sorry — at least once and fulfill all of their conditions. There's no special bonuses to this, but a yellow name run can be considered a reverse genocide run; while in the latter you intentionally find every last monster in order to hunt them down and slaughter them, in the former you intentionally find every last monster in order to be nice to each one individually.
    • Also mocked with the trophies in the PlayStation 4 and Vita versions, see Effortless Achievement below.
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    A 
  • Abandoned Catchphrase: Flowey’s catchphrase, “In this world, it’s kill or be killed!” He says this multiple times during the game. However, after the fight with Asriel Dreemurr, he changes to “”Don’t kill, and don’t be killed.” Even after he turns back to Flowey again, it’s implied that he’s changed and doesn’t believe this anymore.
  • Abandoned Hospital: The True Lab, the last main area in a True Pacifist Route. Although it’s an ad-hoc research facility, it’s still the site of medical experiments Gone Horribly Wrong.
  • Abandoned Laboratory: Also the True Lab. It’s an old lab of Alphys's that has fallen into disrepair. It isn’t entirely abandoned, but it looks the part, and is filled with all the horrors you’d expect from a place like it (everything they could put in a game ostensibly for kids, anyway).
  • Abandon Shipping:
    • Because one of Mettaton’s quiz show questions asked if you would smooch a ghost, there have people who shipped Mettaton with Napstablook. However, a trip to Mettaton’s old house with the Mystery Key and a call with Undyne reveals they are cousins. People not into Incest Yay Shipping dropped the ship.
    • After the 1.01 patch in 2016, new content strongly implied Sans and W.D. Gaster to be related in some way, possibly as family. This caused many people to jump off the Sans x Gaster ship.
  • Ability Mixing: When two or three monsters attack at once, their attacks mix together. Migosp and Jerry are built around this.
  • A Bloody Mess: Sans, the Final Boss of the Genocide Route, bleeds when you kill him, despite the fact that it’s been established that monsters are made of magic and don’t bleed. It is often theorized by fans that this is actually ketchup, because he can be seen chugging a bottle of ketchup at Grillby’s if you decline from using it yourself. Others theorize that he’s not even a monster, in which case the trope is subverted.
  • Aborted Declaration of Love: In the fight with Mad Mew Mew, they reveal that they love Undyne, but she already loves Alphys and they don’t want to interfere with that.
  • Above Good and Evil: You, the Anomaly. After all, this world only exists for your pleasure, doesn’t it? Despite this, the game will try very hard to convince you otherwise on the Genocide Route.
  • A Boy and His X: The Boy is Asriel and the X is Chara. However, the heartwarming story happened in the past of the game, and it has ended in tragedy.
  • Absolute Xenophobe: The humans of this world in general seem to be this. The War of Humans and Monsters was caused because the humans were paranoid about monsters stealing their souls, and according to Asriel, Chara hated humanity. Frisk seems to be the only exception (on the True Pacifist Route, anyway...
  • Abstract Apotheosis: Chara. At the end of the Genocide Route, you’ve brutally murdered everyone in the underground and reached the maximum LOVE. You’re officially an Omnicidal Maniac and you’re as far gone as you can get. And now, you’ve turned Chara into the Fallen Child. They explain how you showed them their purpose: power. You two killed everyone to gain power, because you consider yourself Above Good and Evil. Chara is now the embodiment of death, power, and grinding.
    Chara: HP. ATK. DEF. GOLD. EXP. LOVE. Every time a number increases, that feeling...That’s me. Chara.
  • Absurd Cutting Power: You can destroy the world with a stick, toy knife, glove, ballet shoes, notebook, frying pan, empty gun, or a dagger.
  • Absurdly Ineffective Barricade: Papyrus's fences, which are built with posts much too far apart to stop you. One hopes that they're close enough together to at least stop Papyrus, given how much bigger he is than you, but since he's a skeleton it's unlikely.
  • Absurdly High Level Cap: The level cap is 20, in a game where it's entirely possible (and encouraged) to win at level 1. Getting to the maximum level requires killing every enemy in the game, including the Final Boss, at which point there's no one left to fight.
  • Absurdly Sharp Blade: You can destroy the world with a dagger.
  • Abuse Mistake: The first time you’re chased by Undyne in Waterfall, you get away by hiding in some grass. Undyne will search for you and thrust her hand forward, only to grab Monster Kid instead. Many people playing for the first time mistake this for Undyne hitting or even killing Monster Kid.
  • Accentuate the Negative: It’s pretty common on the Internet to find people saying, “Undertale was great, but the fandom ruined it for me.” Undertale does have a fairly vocal toxic part of the fandom, but there is lots of clean, good, and original fan works as well.
  • AcCENT upon the Wrong SylLABle:
    • The Temmies speak in this manner, which is an impressive feat given that they only get speech blips instead of voiced lines. This is probably the least weird thing about their speech patterns.
    • In a random event in Snowdin, you can get called and hear the "Wrong Number Song". It isn't voiced, and the simplistic nature of the composition makes it difficult to tell where the downbeat is. If one assumes the song starts on the downbeat, the word emphasis gets very strangenote :
      Oh it's the wrong numBER, THE wrong number song. We're veRY veRY sorRY that we got it wrong!
    • During Mettaton's musical number, again only clear via the music:
      Mettaton: They'll put you / in the dunGEON...
  • Accessory-Wearing Cartoon Animal:
    • There’s a mouse in Snowdin Town only wearing a scarf.
    • In a puzzle room in Hotland, there’s a fox (head) that wears sunglasses.
    • There’s a cat monster in MTT Resort wearing shoes and glasses.
    • Snowdrake’s father in MTT Resort wears glasses, a bow tie, and shoes.
    • A mole can found in the resort wearing a hard hat.
    • Chilldrake, an enemy who only shows up if Snowdrake dies, basically looks like Snowy wearing sunglasses.
    • Gyftrot wears all sorts of things, though this isn’t his choice.
    • So Sorry can wear random hats if you encounter him.
  • Accidental Art: If you beat the credits, opening the Mysterious Door, you can find the Annoying Dog’s hideout, where apparently it accidentally programmed an entire game through a program that translated its barks.
  • Accidentally Broke the MacGuffin: If you kill Asgore, his SOUL appears right in front of you, for you to absorb and leave the Underground... only for Flowey to destroy it right in front of your eyes. Many players are understandably ticked off about this. Bonus points on a Genocide Route, where Flowey is trying to convince you he’s useful.
  • Accidental Murder:
    • Some of the monsters are really not trying to kill you in their encounters, but their bullet patterns happen to be lethal (at least to humans, which you are).
    • Toriel only wants to scare you into backing down and returning to the house in the Ruins, as she plans to destroy the entrance to Snowdin to protect you from the monsters. She will react with My God, What Have I Done? if she accidentally kills you during the fight with her, but it's only noticeable for a split second before the game cuts to the Game Over screen.
    • Players experienced with Role-Playing Games in general will get a rude awakening during the first boss fight. The standard method of 'proving' your strength/worth is to beat the opponent to a sliver of their health, then spare them. The first boss is designed to be instantly killed the moment her health reaches about 30%. Most players feel like a scumbag, reload, and then get another rude awakening, unless they've been going for the Kill ’Em All route.
  • Accidental Public Confession:
    Bratty & Catty: Me? Yeah, I LOVE cats?
    Bratty: They’re, like, SO tasty!!
    <beat>
    Catty: Bratty NOOOO!!!
    Bratty: I’m just kidding! Kind of.
  • Accuser of the Brethren: Toriel is not happy with Asgore. AT ALL.
    Asgore: Tori...you came back!
    Toriel: Do not Tori me, Dreemurr! You pathetic whelp. If you really wanted to free our kind, you could have gone through the barrier after you got ONE SOUL, taken six from the humans, then come back and freed everyone peacefully!
  • A Chat with Satan: Chara. They are Frisk’s Shadow Archetype and seem to think they know your dark side quite well. They do give a test of character of sorts with the ERASE or DO NOT choice, as well as the choice to sell your soul. If that wasn’t enough to convince you, they even call themself “the demon that comes when people call [their] name” if you complete two Genocide Routes.
  • Achievement Mockery: Toby makes his disdain for achievements and trophies very obvious in the Sony systems’ trophy list. Once you earn them to make them visible, that is.
  • Achievements in Ignorance:
  • Achievement System: In the Sony versions of Undertale. Not that it’s taken seriously.
  • A Child Shall Lead Them: Frisk can choose to be the monsters ambassador to the humans. It is implied that Chara could have been too if they didn’t kick the bucket.
  • Achilles' Heel: Mettaton in his normal form is impossible to damage, and his lasers can immediately halve your health (and possibly more). However, just flip that switch, and he goes into his EX form. It does have flashier attacks and looks fabulous, but he can’t hit nearly as hard, you can now dodge his attacks, you can damage him, and he has insufficient power supply. Heck, even his EX form has an Achilles' Heel: his heart-shaped core is his weak point.
  • Acme Products: There are a lot of MTT-brand products in the underground.
  • Acquired Situational Narcissism: In Mettaton’s old house next to Napstablook, you can find his diaries, which shows that he was rather devoted to his friends in his old life, but then he met Alphys and almost forgot about his friends. Thankfully, he reaches out to them in the Golden Ending, and assembles a quartet with Napstablook, Shyren, and maybe Burgerpants.
  • Action Bar: In the battle menu, there is one at the bottom, showing the FIGHT, ACT, ITEM, and MERCY options.
  • Action Commands: When attacking, an eye-like meter will appear over the menu, and a bar will run across it. Confirm the attack when the bar is near the middle, and you do more damage. Don't press anything, and you won't attack. Most weapons change this system a bit, such as requiring mashing of the attack button to deal more damage (Tough Glove) or sending multiple bars at once (Ballet Shoes).
  • Action Figure Speech: Some of the mouthless NPCs do this.
  • Action Girl: UNDYNE. Everything. About. Undyne. She is living anime. She’s a foreboding Black Knight at first, but once she reveals herself, she reveals herself to be a hammy and Hot-Blooded anime protagonist who does everything with passion! Even cooking is done with monologuing! Papyrus lampshades this in one phone call.
    • Frisk and Chara could fall under this category too, depending on what gender the player sees them as.
  • Action Politician: Asgore. As one can see in the intro, Asgore was on the front lines during the War of Humans and Monsters. He also trained Undyne himself and is even willing to face you himself at the end of the game. Possibly subverted, as it is implied that he wants you to kill him. It’s also implied that he’s even more powerful than that, because his check text says he has 80 ATK and 80 DEF. That’s enough to kill Frisk at LV 20 in two hits! Not only that, but he is also able to interact with the battle interface. The only other people who we know can do that are Photoshop Flowey, Asriel Dreemurr, and Sans! If he didn’t hold back, he could easily be the most powerful monster in the Underground.
  • Action Survivor: If the player does a Low-Level Run, Frisk qualifies. Frisk is only a kid implied to be around 8. They’ve also survived a world where Everything Is Trying to Kill You, has faced down Photoshop Flowey and the Amalgamates, and can become the monsters ambassador to the humans.
  • Activist Fundamentalist Antics: A unique example. “Kill or be killed” is practically Flowey’s catchphrase. He doesn’t go around telling everyone his philosophy, but he goes to pretty extreme lengths to convince the human of this.
  • Actually Pretty Funny: Quite a few examples.
    • During the first encounter with Papyrus, he is trying to get Sans to work on his puzzles, but Sans keeps blowing him off hilariously.
    Sans: hey, take it easy. i've gotten a ton of work done today. a skele-ton.
    Papyrus: SANS!!!
    Sans: come on. you're smiling.
    Papyrus: I AM AND I HATE IT!
    • During Mettaton’s quiz show, when he finds out that Alphys is helping you, he asks you a question that “[she’ll] be sure to know the answer to,” which is, “Who does Dr. Alphys have a crush on?” If you answer yourself, Mettaton says this:
    Mettaton: ... SERIOUSLY? MY MY... HOW CONCEITED CAN YOU GET...? I LOVE IT! AND WHILE YOU ARE COMPLETELY WRONG, YOU DESERVE SOME CREDIT. I'VE SEEN HER WATCH YOU ON HER COMPUTER SCREEN. SMILING WHEN YOU SUCCEED. SHRIEKING WHEN YOU FAIL. AND ALWAYS, ALWAYS, WHISPERING... "NO! WRONG! YOU HAVE TO GO THAT WAY!" IN ITS OWN WAY, IS THIS NOT LOVE?
    • If you ask Gerson about the Delta Rune, he calls Asgore Dreemurr “Ol’ King Fluffybuns.” If you ask him about it, he says it’s a great story, but he doesn’t remember it. However, you can get the story in the Playable Epilogue of the Golden Ending: One day, during the King and Queen’s monthly address, after Toriel finished her part, as she was passing the microphone to Asgore, she accidentally said into it, “Your turn, Fluffybuns.” As the whole audience burst into laughter, Asgore rose his arms to silence the room, then said, “I, King Fluffybuns...”
    • During the fight with Sans on the Genocide Route, there is a Mid-Battle Tea Break, and he offers to spare you, saying he still believes there’s a good person inside of you. If you accept the offer, he commends you for it and asks for a hug...before slamming you with an undodgeable attack that kills you, the same way you killed his brother. He then speaks to you in the Game Over screen and drops his iconic line: “geeettttttt dunked on!” It’s supposed to trigger a Rage Quit and you still have to redo the fight, but the music and the way the line is delivered is drop-dead hilarious.
  • Actual Pacifist: The player can be this if they choose to. After all, that’s the whole point of the game: you can defeat your enemies by talking instead of fighting. While you do have to fall into Technical Pacifist territory to get the Neutral Ending (and by extension, the Golden Ending) due to the fact that you have to attack Asgore to progress, you load your SAVE file afterward, which undoes the fight with the king, allowing you to beat the game without harming anyone.
  • Adaptation-Induced Plot Hole: An extremely minor example. Since the PlayStation versions replace all dialogue referring to the F4 key, the "secret fourth frog" Brick Joke during the Playable Epilogue comes off as completely random. Then again, if you haven't been told about the "secret fourth frog" in the early game, it's absurdly unlikely that you'll find it to begin with, so it's even less of an issue than one might think.
  • Added Alliterative Appeal: During Papyrus' spiel about capturing a human, he gets so excited he breaks into alliteration.
    Papyrus: POWERFUL! POPULAR! PRESTIGIOUS!!! THAT'S PAPYRUS!!!
  • Adipose Rex: Asgore is rather stout, as can be seen in the Golden Ending when he’s not wearing his armor. However, he subverts most of the usual connotations that come with this trope. He is not only The Good King, but an Action Politician who is one of the most powerful monsters in the Underground.
  • Adjacent to This Complete Breakfast: The E3 announcement of Undertale being ported to PlayStation 4 and PlayStation Vita jokingly stated that the game was part of a complete breakfast◊ which consisted of cereal, waffles, and four glasses of orange juice.
  • A Dog Named "Dog":
    • According to Word of God, the name of the child who you hang out with in Waterfall is actually “Kid.”
    • In Snowdin, there is Snowdrake and Chilldrake. They’re made of snow and ice, which makes them a snow drake and a chill drake.
    • The Temmies. It seems to be the name of the species and every member of it (except for Bob).
  • Adopted into Royalty: When Asriel found Chara after they fell into the Underground, he brought them to his parents, and Chara was adopted into the royal family.
  • Adopt the Dog: The player can do this at several points, depending on how they play.
  • Adorable Evil Minions: The Snowdin Canine Unit. They’re just a bunch of fluffy dogs with armor. Two of them aren’t even capable of coherent speech! They also love to play and will immediately stop fighting if you throw the Stick. You defeat every single one by PETTING them. You get to literally Pet the Dog(s)!
  • Adorkable: Many character has shades of this, but among the main characters are:
    • Toriel. You first meet her when she saves you from Flowey at the very beginning of the game. She puts the “dork” in Adorkable when you speak to her as she is reading. She excitedly tells you all the things she wants to do with you, mentions how she’s always wanted to be a teacher, and is excited to tell you exciting snail facts. She’s a Pungeon Master, too!
    • Papyrus, Sans’ very loud brother with a Small Name, Big Ego who hopes to be in the Royal Guard one day. Because, despite those, he's genuinely nice, friendly, and naive. He's the only character who is incapable of killing you (not from lack of strength, but he's too nice). He also sleeps in a racecar bed, owns a bunch of action figures, believes in Santa, loves cooking spaghetti, and if you befriend him he will do everything he can to get you and Undyne to be friends.
    • Undyne has shades of this, due to the fact that she genuinely believes anime is human history, and bases her entire way of fighting and lifestyle on it.
    • Alphys is the most Played Straight example. She's incredibly awkward and shy, and is very uncomfortable speaking to most people. She's also extremely friendly once she comes out of her shell. Just look at her talk about Mew Mew Kissy Cutie!
    • King Asgore Dreemurr, who may look scary and intimidating at first, but is, in Papyrus' words, a "big fuzzy pushover". He loves having tea, gardening flowers, and is a notorious Giver of Lame Names.
  • Adult Fear: All runs of the game thrive on this trope:
    • The story of Asgore's children. Two siblings, one Happily Adopted, the other a prince with Big Brother Worship. At first, one merely has a nasty sense of humor about getting their dad sick with buttercups, but then they get the idea to poison themselves in a failed Thanatos Gambit that results in both of them getting killed in the same 12 hours – one dying of poison while the other runs away and comes back fatally injured. Neither of them seemed older than around 13 at the time.
    • Asgore, after that horrible night, declares war on the humans again, saying that any that fall into the underground should die, and their SOULs used to destroy the barrier. This breaks up his marriage with Toriel because not only is it He Who Fights Monsters, but Toriel points out that since a monster can cross over when bonded with a human soul, Asgore would simply have to kill one human, cross, and collect more SOULs from the surface where all humans die eventually. Instead, Asgore lied to the people of the Underground so that he wouldn't have to dirty his hands directly and let them do the killing. He really doesn't want to kill anyone, though, so he was stuck between leaving his people stuck underground forever, risking the same fate as his son after murdering another child, or murdering seven children, all after having lost his entire family within the span of a few months at maximum.
    • Toriel saves the player character from Flowey, but she doesn't know that Flowey is composed of the remains of her son Asriel. She also is Properly Paranoid about wanting you to stay in the Ruins, since Everything Is Trying to Kill You, if the box of mismatched child shoes is any indication. In any Neutral ending where she's alive, she quickly undoes the aforementioned "any human who falls into the Underground must die" policy.
    • Alphys's experiments for the subjects' families. To clarify, dozens of comatose monsters came to her lab, and she was trying to see if they could hold determination. It soon was an experiment Gone Horribly Wrong for the subjects, and their families asking where they were. Alphys was too cowardly to admit that she had created the Amalgamates, as well as Flowey.
      • It's something of a three-fold example, as first the amount of time Alphys was taking led to the monsters wanting their family members' remains back, so they could at least have a proper funeral. However, Alphys then told everyone that the monsters were actually alive, getting their hopes up... but then they became the Amalgamates, so she kept them in her lab, making it look like she was holding everyone's relatives hostage for no reason. And the amalgamates themselves fell down one day and suddenly woke up literally melded together with other monsters and trapped in a dark, terrifying lab, desperate to see their families and completely unaware of the events that led up to this point. It was extremely horrific for everyone involved.
    • If you killed Snowdrake on a Neutral run, you meet his father way later in Hotland, and what was a bit of backstory about him being rebellious after the death of his mother is replaced by frantic pleading if you've seen his son, who's disappeared completely and has him terrified.
    • If you choose to kill Papyrus, Sans will disappear from the game completely until the very end where he takes it upon himself to judge your actions. Depending on what route you're on, his reaction will range from incredibly chilling Tranquil Fury to, as he puts it, giving you "a bad time". Keep in mind that not only is Sans older than Papyrus, but there's enough evidence to make many fans believe that he raised him. There's one particularly difficult conversation where Sans is going over your EXP and LV and asks you outright why you killed his brother if you're as good and loving as you say you are.
    • Asriel at the end of the True Pacifist Run asks Frisk not to tell his parents where he is or that he will revert into being a soulless flower. In the meantime, Toriel is still angry at Asgore and sends him one Death Glare after another, though Alphys hopes that they will get back together. It's highly likely that if Frisk had the option of telling Toriel and Asgore that Asriel is alive and was the flower, they could have reconciled or at least formed an Enemy Mine to retrieve their son, but then they would have had to cope with the knowledge that he had been suffering this whole time and is now completely unable to feel love and compassion.
  • Advancing Wall of Doom: Mettaton’s multicolored tile puzzle has a firewall that moves toward you. However, if you don’t win, Alphys will deactivate it before it actually harms you.
  • Aerith and Bob: The monsters' names. Toriel, Asgore, Undyne, Sans, Papyrus, Napstablook...Aaron and Jerry. And, of course, there's the Temmie that is literally named Bob. And, of course, the player character can be named whatever you want, although the Canon Name of Frisk and Chara is an example as well, as Frisk barely sounds like a name, while Chara is a more reasonable-sounding one (even if it's actually short for "Character," it sounds similar to the actual name Kara).
  • Aesop Amnesia: Justified. Resets make characters forget their character development.
  • A Father to His Men:
    • Undyne is a female example. She makes it very clear how much she cares about Papyrus, though the relationship is more similar to brother and sister than mother and son. If you kill Papyrus, Undyne drops the monologues, and coldly promises to kill you. The only character who cares about Papyrus more is Sans. She also cares very much about the Snowdin Canine Unit and Guards 01 and 02.
    • Asgore also fits. He trained Undyne himself from her childhood and is very caring towards her (and pretty much everyone). There isn’t a single monster who dislikes him, except for Toriel.
  • Affectionate Gesture to the Head:
    • Toriel gently pets the player on the head when they first enter her house and she shows them their new room. No surprise why fans quickly took to calling her Goatmom.
    • Parodied during the "date" with Alphys. Undyne hugs a depressed Alphys and pats her on the head... then tosses her into a trash can like a basketball before shouting an encouraging speech at her.
    • In an Easter Egg in the True Lab, lying down in one of the beds causes a ghostly creature to appear beside it, and slowly reach out to Frisk...To reach down to tuck them in, and pat them on the head. It would be cute, if it weren't creepy as well.
  • After Boss Recovery: Toriel, Papyrus, both forms of Undyne, Mettaton EX, and the Mad Dummy all have a Healing Checkpoint right after their fight.
  • Agent Peacock: Mettaton is flamboyant and super vain, and he loves attention, glitz, and allegedly fashion, throws out terms of endearment in most of his dialogue, and spends one scene in a dress. He's also surprisingly clever, cool under pressure, and was, according to Alphys, additionally programmed to be a ruthless human-killing machine. The last part turns out to be a lie for complicated reasons, but he can be pretty ruthless when he wants to be. This trope is thoroughly ramped up when he switches into his EX form, switching him looks-wise from a rectangle with an LED screen and a wheel to a smirking Bowie-esque Pretty Boy in Combat Stilettos, giving him stats and attack patterns that make him one of the game's tougher bosses.
  • Aggressive Categorism: Undyne has a very skewed view of humans before the player sets her straight.
    Undyne (as an amnesiac Lost Soul): All humans must die!
  • A God Am I:
    • On your first Neutral Route, Flowey, upon absorbing six human souls, claims that absorbing one more (the protagonist's) will allow him to become God.
    • On a True Pacifist Route, Flowey absorbs the six human souls plus every monster soul in the Underground (which was said on an easily-missed Pamphlet Shelf to be equal to the power of one human soul), and takes on a One-Winged Angel form he dubs the 'Absolute God of Hyperdeath.
  • Ain't Too Proud to Beg: Flowey has no qualms about begging desperately for his life at the end of the Genocide Route. The Fallen Child doesn’t listen.
  • Alas, Poor Scrappy: Most people playing the game aren’t particularly fond of Flowey. They aren’t exactly unjustified, either. However, at the end of the Genocide Route, seeing him beg desperately for his life and seeing the Fallen Child repeatedly slice his face in half is still quite disturbing to many players.
  • Alas, Poor Villain: See Alas, Poor Scrappy above.
  • Alchemic Elementals: The guardian of the watery zone of Waterfall is a fish-woman named Undyne.
  • Alignment-Based Endings: There is the Neutral Route, the True Pacifist Route, and the Genocide Route.
  • A Lizard Named "Liz":
    • When Asriel came back to life, he named himself FLOWEY. It seems Asgore’s Giver of Lame Names trait runs in the family...
    • In an alleyway next to MTT Resort, you can find a shop, in which one of the shopkeepers is a cat named Catty.
    • Tsunderplane is a tsundere plane.
    • Pyrope. Pyro is a root that means “fire.” In other words, Pyrope is a fire rope.
    • There is an NPC in Snowdin Town who throws ice into a river. In the Playable Epilogue, when he finally leaves his post, he calls himself “Ice Wolf.”
    • A fire monster in Hotland is named “Heats Flamesman.”
    • In the Ruins, there are frog monsters called Froggits and carrot monsters called Vegetoids.
    • All members of the Snowdin Canine Unit.
  • Allegorical Character: Chara is supposed to represent a typical RPG player. They kill in order to gain power, are happy to see their stats increase, and once they are done with a world, they move on to the next.
  • Allergic to Evil: A monster's body and soul are interconnected, so their mental state is directly tied to their physical strength. As such, a person with a particularly strong Killing Intent is capable of dealing incredible damage to a monster's body. This is why killing monsters makes you stronger; you become more distant emotionally and treat them more as obstacles.
  • Allergic to Routine: Flowey explains his history in full towards the end of a Genocide Route: with the ability to SAVE, he was able to experience all possible outcomes of every scenario. He helped everyone, he killed everyone, he won every contest and lost every contest... until, after untold resets, he just ran out of things to do. It wasn't until the Player Character fell into the Underground and overrode his SAVE power with their own that he became truly interested in the outcome of events. The same urge is part of what likely drives you, the player, to start a Genocide Route of your own...
  • All First-Person Narrators Write Like Novelists: In-Universe and downplayed example. Alphys writes in a rather dramatic way in her entries in the True Lab. It’s almost like she knows the Player Character will read them. Justified, however, as Alphys watches anime, which is well-known for its over-dramatization.
  • All for Nothing: Killing any main character could count as this, especially on the Genocide Route.
  • All Girls Want Bad Boys: Tsunderplane still likes you on the Genocide Route. Suffice to say, this does not end well for her.
  • All Is Well That Ends Well:
  • Alliterative Name:
  • All There in the Script:
    • The sprite filenames are labeled "gasterblaster" for one type of bullet used by a certain late-game boss. It's the laser cannon that Sans uses, and is one of the clues hinting at a relation to W.D. Gaster.
    • The canon name of Photoshop Flowey is only given within the credits.
  • Almost Kiss: Undyne and Alphys almost get to share a kiss near the end of the True Pacifist run, but Toriel interrupts them because the human is watching.
  • Alone with the Psycho:
    • During a True Pacifist run, the fight with Undyne can come off as this, even with other characters potentially watching. You have the option to flee Undyne if your heart goes back to normal, and she will chase you and keep challenging you. While in other runs, such as No Mercy, this comes off as Properly Paranoid on Undyne's part, in True Pacifist, you are an actually harmless child and she is hunting you down. Sans notices, but all he does is briefly distract Undyne by sleeping on the job, allowing you to make it to Hotland.
    • During a genocide run, Undyne and the player switch roles in this trope. A few times.
    • Near the end of a run, you get to play this trope again against Sans. Although you're still the psycho, you'll quickly realize it's not Sans who's trapped with you...
  • Aluminum Christmas Trees: The Glamburger is a twofer.
    • The Info text specifies the glitter and sequins on it as edible. Edible glitter and sequins do exist, but they're usually used on cakes and similar sweets.
    • The most expensive burger in the world is called a Glamburger, though it uses gold and caviar rather than glitter and sequins.
  • Always Accurate Attack:
    • Unlike most RPGs, there's no accuracy or evasion stats; if you pull off the attack, it will hit, though a few enemies can No-Sell it. This is because, while monsters attack your SOUL with dodge-able bullets, you attack them directly and they don't try to avoid it (likely out of Mook Chivalry or ignorance to human methods of fighting), which is just one thing that makes you very powerful if you take advantage of it. In fact, one of the most startling things in the game is when Sans, the final boss of the Genocide Route, casually dodges your attack. No, he doesn't block it or anything, he just steps to the side as if it was nothing. After all, why should he stand there and take it if everyone else who did so died?
    • Flowey's signature attack is to surround you with a dense ring of bullets that is completely impossible to escape, which he then contracts either very quickly or painfully slowly. He uses it in cutscenes many times throughout the game, but something always stops him from dealing the killing blow, at least when he's using it against you. He turns it up to eleven in one cutscene by instead shrinking the playfield to the exact size of your heart, so you cannot move.
    • The No Mercy route's final boss' last actual attack, smacking you silly against the borders of the box, also deals unavoidable damage. Unfortunately for him, it also can't kill you. He also has a box-covering attack which he uses when you try to spare him, and this one will kill you.
    • The True Final Boss has an attack that covers literally the entire screen. Fortunately for you, it's only a HP to 1 attack. He will later pull off another unavoidable attack that reduces you to a fraction of a HP.
      • It's actually possible to dodge the former attack, but it requires some luck and involves dodging giant projectiles during a moment when you can't even see anything and your movement range is limited, so it may have been intended to be unavoidable. The latter literally covers your entire movement area, so there's no dodging it.
  • Amazing Technicolor Battlefield: The first phase of the True Final Boss of the Pacifist Route has the background as a changing wave of colors while the boss moves around it. The second phase has the boss's wings with moving colors as well.
  • Ambiguous Gender: The player character is only referred to in gender neutral pronouns.
  • Ambiguous Situation: With the uncertain level of Player and Protagonist Integration, "you" becomes a rather imprecise pronoun, and a few conversations take advantage of this.
    • At the end of a Neutral run, if you spared Flowey, he'll speak to you. But it's not clear whether he's speaking to "you" as in the player, or "you" as in the player character. It is at least clear that he's not speaking to the Fallen Child, as if you do this enough times, he'll derisively point out that they were his Only Friend, and you aren't going to change that.
    • If you boot up the game again after a True Pacifist run, Flowey will appear and beg you not to True Reset. Again, it's unclear which "you" he's addressing: "you" the player, or "you" the Fallen Child. But since he pleads you to "let Frisk live their life", we at least know he doesn't mean them.
    • At the end of a Kill ’Em All run, the Fallen Child will speak to you, giving you credit for their decision to devote their resurrection to killing. And if you boot the game up afterward, they'll eventually ask for your SOUL in exchange for resetting the world. Again, it's unclear whether they mean "you" the player, or "you" the player character. It's even possible they're invoking this trope, meaning "you" in one way and hoping you'll interpret it in the other.
  • Ambiguous Time Period: 201X doesn't give the greatest sense of time, except telling us that the time period the game takes place in is in the New Millennium. This also, probably intentionally, muddies the water about how many decades or even centuries have passed between the opening cutscene and the beginning of the game.
  • Amen Break: "Your Best Nightmare/Finale".
  • An Aesop:
    • Undertale has many, many things in it worth learning from, some of them more subtle and open to interpretation than others, but the most clear and consistently-reiterated one is it's possible to love something too much, or to love in a detrimental way. Toriel is willing to trap the Child in her home in the ruins forever out of a misguided but well-intentioned attempt to protect the child from a hostile world. Flowey at one point addresses the player by directly referring to the story as a game and refusing to let the player "win" out of a fear that they'll move on from Undertale and leave everyone behind — his objective is to keep you playing the game over and over, even going so far as to give you advice on how to get better endings. During the Final Boss fight against Asriel, it quickly becomes clear that he just wants to keep fighting you over and over (as you can't die) because it means he can be with his beloved lost sibling. If you complete the game on a perfect run and get the best ending and then try to start a new game, Flowey will turn up and chastise you for being willing to rip all your friends away from their happiness just so you can revisit a world you really should just be done with. The "No Mercy" run is the ultimate logical conclusion of this message: the player is so in love with playing Undertale that they're willing to become a full-bore Omnicidal Maniac just to see some new game content.
    • Another Aesop is that violence isn't always the answer and forgiveness can change a life. In the True Pacifist ending, Frisk sets every single main character on a happier, clearer path than they were on in the beginning: Sans regains his hope for his own future and breaks through his apathy, Papyrus realizes that he doesn't need to live up to other people's standards to be loved, Undyne accepts that her violent inclinations may not always be the answer, Alphys decides for herself to work towards self-confidence and being worthy of forgiveness, Toriel learns to let go a little and let people live their lives (even if it hurts sometimes), and Asgore stops being resigned and takes control of his own fate for the first time since his children died. Even Asriel begins to see the point in letting people live their lives without interference, and all of this is due to Frisk's endless kindness and mercy jarring against their own views of themselves, each other, and the world around them.
    • On the other hand, the game also notes that sometimes violence really is the only answer — pacifism is only possible in a situation of absolute power, like how the player character has the ability to essentially try again and again infinitely. Even if they are a small child, the fact of the matter is that they can't die unless the player gives up and stops playing the game, and as a result, the battles are reframed to be less "a monster and human fighting one another" and more "a monster versus an immortal". Defending yourself is completely natural, but considering how far above the average monster you are in terms of sheer power, killing them would be excessive. Despite this, there are some situations in which the person you're fighting just will not listen to reason, such as the final fight against Asgore, and as a result, the only reasonable option short of letting them kill you is putting up a fight. Sans notes in the neutral path judgement that the stuff you've done is all logical from your perspective as a human being with a will to live. Asriel also comments on this in the post-game Pacifist walkthrough if you can find him, saying that always being pacifistic won't work in the real world. Using force to solve a problem while you hold all the power is at best selfish and cruel, and if you can find a solution where everyone can be happy through non-violence, then you should strive for that instead – even the aforementioned situation where you have to fight eventually reaches a point where you can de-escalate.
  • And Another Thing...: A humorous variant by Sans when he is about to leave during the cutscene at Grillby's:
    Sans: by the way... i was going to say something, but i forgot.
  • And I Must Scream:
    • In the Pacifist run, the True Lab's Amalgamates cannot talk at first, and the player character doesn't even know their names. Their only contact with the outside world is Alphys, who feeds them and hides them from their families.
    • This happened to Flowey when he first woke up as a flower. He woke up inside a flower, unable to move or feel emotions, with no memory of what happened to him. When he called out for his mother and father to help him...
      "But nobody came."
    • The six SOULs that Asgore keeps in his palace. They were once children that Toriel raised, and they are not allowed to go beyond. During the fight with Photoshop Flowey, he uses them to attain godlike status, but they help by healing the player character and rebelling against their owner.
    • In the best ending, every monster plans on vacating the underground to live on the surface. Except for Asriel, who will eventually transform back into Flowey. Keep in mind, Flowey can apparently live a long time (perhaps forever), and it's not specified whether or not he can (or even if he wants to) get out of the Underground. He can move around on his own, that much is certain (he can be briefly seen following Frisk at several points in the game if Frisk backtracks), but it's not known to what extent.
  • Animal Motif: While Toby Fox never physically appears in the game, his avatar does. Known as the Annoying Dog, he primarily bothers Papyrus and notably absorbs the Legendary Artifact. He can also be found as an Easter Egg after the game's credits as a pomeranian sleeping in a room representing his office next to his computer.
  • Annoying Video Game Helper: invoked Given her attitude towards the player, her role, her Punny Name, and the game's nature as a Deconstructor Fleet, Toriel is likely intended as a satire of the obligatory annoying tutorial character that tells you stuff that should be obvious even to a goldfish. She literally holds your hand through at least one puzzle! However, unusually, it's all Played for Laughs and kinda heartwarming more than it is annoying.
  • Antepiece: An odd example. Whenever your SOUL mechanics are changed, you almost always have a chance to get used to them before the bosses throw their most dangerous attacks. Same with attack colors; blue attacks are first used by a mini-boss that is explicitly said to only be able to see moving things, while orange attacks are first seen as field obstacles. The only exception is the final battle of the Genocide run, who will happily turn your SOUL a new color and use difficult attacks despite the fact that you don't encounter this mechanic anywhere else in the run.
    • Probably intentionally done with the aforementioned final battle of the Genocide run (against Sans) to punish the player for doing a Genocide run before a Neutral or Pacifist run. In those runs, the same mechanic is introduced early on in the fight with Papyrus, and it follows the pattern as above. In a Genocide run, though, you heartlessly murder Papyrus without ever fighting him, and therefore the battle with Sans will likely catch you completely off-guard. Likely also due to the fact that Sans is the only Combat Pragmatist in the game with genuine reason to kill you, while it's strongly implied that, even though the others say they want to kill you, most of them have never killed and don't fully understand or enjoy the reality of what it entails.
  • Anti-Climax:
    • Played for Laughs with the instant noodles: if used in battle, they take a hilariously long time to prepare with no option to skip the text, including taste-testing them and adding the flavoring packets... all this for a healing item that restores 2-4 health, not enough to be worth it even on a Pacifist run (where you have really low max HP).
    • Learning the secret of Sans's bedroom, which is incredibly obtuse. You enter his room and walk through a long, pitch black darkness for half a minute, a loud droning noise getting closer and closer, suddenly everything goes black... and Papyrus turns on the bedroom light to show that his room is more or less normal (aside from the the self-sustaining trash tornado in the corner) and you've just been running on a treadmill, the entire thing was a prank. Double-subverted, though, because you find the key to the room behind his house where Sans keeps the broken time machine, a hint towards his unsaid backstory that's the reason he has such a depressed disposition.
  • Anti-Climax Boss: Invoked trope: on a No Mercy Run, Toriel, Papyrus, Muffet, Mettaton NEO, and Asgore, as well as several mini-bosses, go down in one hit due to your intense murderous desire. Mettaton NEO is especially noteworthy because it's set up to look like he's going to be an upgraded version like Undyne was.
  • Anti-Frustration Features:
    • The first two bosses, Papyrus and Toriel, can't cause a game over unless you work for it. You have to actively run into Toriel's later attacks, as they swerve to avoid you since she doesn't want you to die, while Papyrus will only bring you to an unconscious state from which you'll wake up in a shed next to the house.
    • In the Snowdin Forest, Papyrus presents three pathing puzzles. The second one, while not being very difficult in and of itself, has a solution that's only slightly less obvious than the other two (due to Papyrus himself changing the solution by rearranging the snow around the puzzle), and if you "absolutely, dapsolutely want the solution", Papyrus will reveal that instead of solving the puzzle proper, you can just flip a switch hidden in a tree.
    • If Papyrus defeats you three times, he'll let you pass without having to fight him again. Though you can continue to fight him if you want. He'll also offer to spare you if you reduce his HP to a certain amount, just in case you still haven't figured out how to spare bosses without fighting them (though this also has the added effect of making killing Papyrus a genuine Kick the Dog act).
    • Your reward for solving the piano puzzle? Infinitely-replicating Dog Residue. If you need money (and have patience), you can repeatedly create Dog Residue and sell it in Temmie Village. Dog Residue also has a small chance of creating Dog Salad, which is a healing item.
    • In Temmie Village, if you buy the "item" that pays for college, it unlocks a new item called Temmie Armor. It has such high defense that Temmie repeatedly insists it makes the game too easy. It's extremely expensive, but the price lowers every time you die, so it's there as a last resort.
    • If you lose during Mettaton's final encounter, you can skip his opening monologue when you retry, which also applies to any subsequent playthroughs. Many other pre-boss cutscenes are similarly cut short if you have to retry, and both Undyne and Asgore conveniently let you save after they give their big speeches.
    • A powerful weapon and armor are sitting out for you to take just before you reach the climax of the game, which is handy if you haven't made any investments in upgrading your gear. Especially nice on a pacifist run, since the next boss is the only one where your ATK stat matters.
    • The "don't step on the leaves" puzzle in the ruins will eventually just turn the entire floor solid if you fail at it enough times.
    • The "change in perspective" puzzle in the Ruins will tell you where the first switch is and clue you into the pillar-hiding mechanic if you walk past the pillar three times without figuring it out.
    • All of Mettaton's methods of tormenting you throughout Hotland won't end up killing you as he intended, even if you fail them. Usually Alphys will sabotage the plan or Mettaton will just decide not to. Especially useful since most of his challenges are downright Nintendo Hard. And in your final confrontation with Mettaton, you find out why. Everything except Mettaton's boss fight was part of Alphys's plan to make you like and rely on her enough that you would choose to stay in the Underground instead of going on to fight Asgore.
    • Toriel is programmed to take much more damage when she's already low on health. It's a straight example if you want her dead, but an inversion if you're trying to spare her (it's likely this was intended to trap players who were attempting to spare her by getting her health low). Though it is less marked than in the Toriel battle, this is true of every boss — likely to prevent the player from doing a Rage Quit if they get killed by a boss with a sliver of HP. It also prevents the player from trying to spare every monster using the health method, just in case they didn't get the memo during Toriel's fight.
    • During the boss-fight against Photoshop Flowey, dying and reloading after saving at least one soul preserves your progress, as Flowey repeatedly saves the game during the fight to cheat. Dying after saving at least one soul will just close the game automatically rather than having Flowey taunt you on every death and retrying skips his boss intro.
    • The pacifist run's final battle against will never take you into a game over screen at all, and has checkpoints every few rounds. It's a good thing for two reasons — the battle is preceded by 10 minutes worth of cutscenes, and there's a portion where you can't heal for 5 turns after an attack that takes most people down to 1HP, and has a pattern that's almost impossible to dodge.
    • In the Genocide run's final battle, you could die in the second stage of the battle by the attacks thrown out from within the player menus. Due to feedback from beta testers, this was changed in the release version so that menu attacks could not take you below 1 HP.
    • The game will hold your hand in some boss fights if it becomes clear that you are having difficulty trying to spare them.
      • If you kill Toriel, Flowey will sarcastically hint that you can go back and try again if you want to save her. If you do this, the battle descriptions will drop very heavy hints about the correct path to get past her.
      • If you try to talk to Toriel three times, the game will outright tell you that TALKing doesn't seem to be the solution.
      • The game will tell you that you cannot escalate Papyrus's fight by ACTing if you continue to try.
      • If you are particularly stubborn in being merciful to Asgore, the game will eventually tell you that "All you can do is FIGHT".
    • The way to the Undyne fight on a No Mercy route from the nearest save point (Temmie Village) is a long way. The first time that you die to this boss, a new save point will be placed on the screen right before the one you fight them on. In addition, when you LOAD after dying, she simply says, "You're gonna have to try a little harder than THAT!" instead of going through her whole speech again.
    • If you do a No Mercy route, you won't be able to get the phone upgrades that let you access your Dimensional Box at any time on the map or give you access to a second Dimensional Box, because Alphys, who provides these upgrades on non-NM runs, doesn't show up in person to meet you due to being an indirect enemy at this point. To make up for this, the physical box will appear in a few places where it won't on other routes, such as the hotel, New Home, and the corridor right before the Final Boss.
    • If you're going for a Pacifist run, you can Flee from any mook you can't figure out how to Spare without affecting much of the ending. All that'll change is that the "Where Are They Now?" Epilogue will show them in imperfect (but still good) callings.
    • This is likely the reason why there's wind blowing in the void that's left after you boot up the game after finishing a Genocide Run — an audio indicator that no, the game did not freeze mid-loading.
  • Anti-Grinding: If you keep killing the Random Encounters, eventually the area will run out of monsters, limiting how much EXP you can earn. However, nothing stops you from repeatedly sparing monsters for Gold, other than the fact that you don't really need to unless you want the Temmie Armor.
  • Anti Poop-Socking:
    • Sometimes the riverperson will remind you to take a break every once in a while.
    • Some characters try to talk you into quitting the game outright. After getting the best ending, Flowey begs you not to reset the timeline. On a No Mercy run, Sans tries his damnedest to make you quit out of frustration (or at least force you to reset to a happier, more merciful route to avoid fighting him), and the Fallen Child would rather move on to other games after destroying the Underground.
    • This gem from Asgore in the Playable Epilogue:
      Asgore: Oh, Frisk, if you're not busy... on nice days, you should walk around and have a good time.
    • One of Toriel's text messages compliments your character for not spending a lot of time using a computer, which is unhealthy.
  • The Apocalypse Brings Out the Best in People: In many ways, the player sees the best side of each character during the No Mercy route, which sees the death and destruction of all things if taken to its conclusion. Papyrus realizes the value of the friends he already has and stops being so desperate about proving himself, then perishes forgiving you and affirming his hope that you can change. Undyne takes a mortal blow for a monster child, then gathers the determination of the entire world to keep fighting until her body gives out, basically becoming one of her beloved anime heroes. Mettaton reveals as clearly as possible his unselfish love for humans and monsters alike trying to talk you down. When Alphys sees what you are doing, she overcomes her timidness and rises to the occasion to save as many monsters as possible from your rampage and becomes a leader and a hero in her own right. Even Sans, implied to know what would happen if you successfully complete the route, finally shakes free of his apathy to battle you head to head with all his incredible strength.
  • Apocalypse How: The Genocide ending starts with you personally going for the extinction of all monsters, and ends with the Fallen Child pulling off something between Total Extinction of the Underground and Universal Physical Annihilation, in one attack. The only thing we know for sure is that, when you load up the game afterward, all that's left is a black void and a howling wind.
  • Apocalyptic Log: Two of them, both in the same hidden location. The True Laboratory has the written logs from Alphys and the audio tapes from before the deaths of Asgore's children.
  • Applied Phlebotinum: A twist revealed near the end of the Pacifist route (but implied in others routes) is that there's actually a difference between being determined as a personality trait and actually possessing a thing called Determination. It's vaguely defined as "the will to keep living; the resolve to change fate" and apparently exists in some kind of distillable form like a chemical or hormone. Humans possess it in abundance and it's the reason you're able to SAVE and never die permanently, while monsters can only handle a certain amount before succumbing to a horrifying Phlebotinum Overdose since they're "made mostly of magic" and lack the physical matter to process it.
  • Arc Symbol: Quite a few.
    • A strange symbol that is composed of three triangles and a circle with what appear to be wings coming out of it repeatedly pops up throughout the game, such as in shops and on Toriel's dress. At the halfway point, it's explained that this is called the "Delta Rune", and is a symbol of monsters since ancient times. According to a prophecy, the winged circle is an "angel" that will free the monsters (which are represented by the triangles) from the Underground. Others believe it's an angel of death that will slaughter them all. At the end of the True Ending, Asriel transforms into a horrific creature that bears a striking resemblance to the Delta Rune. At the end of the fight, he shatters the barrier and fulfills the prophecy. The "Angel" is also noted to have been to the surface, returning to free the monsters. Asriel is the only monster in the underground who had been to the surface, after the barrier was placed. Considering that he died soon after, it also makes him a literal angel upon his return in the game, when he shatters the barrier.
    • There's another recurring shape that — for the most part — isn't seen until late in the game. The unique shape of the determination extractor pops up in Photoshop Flowey's body, Sans's Gaster Blasters, and Asriel's Hyper Goner. And, for whatever reason, Gyftrot's face as well. It appears to be based on a goat skull.
    • Eyes seem to be significant, with the left eye possibly representing violence. The meter that appears when attacking is shaped like an eye, and on the No Mercy route, we not only see that Mettaton appears to be missing his right eye, but both Undyne and Sans get a brightly glowing left socket. When Sans is joking around on a normal route, he will wink by closing his left eye, but when he winks during his boss battle, he closes the right eye, leaving his left open. And finally, the player's attack option is always on the left and mercy is always on the right.
    • Flowers, although they don't carry the usual connotations of innocence (as Flowey demonstrates). The golden flowers are especially important. Their bright color contrasts with the muted palette; they have significance to Toriel and Asgore, and they twice save the Player Character from a fatal fall. (The second time they appear in a very unlikely place.) Flowers seem to be associated with wishes, e.g. the echo flowers, and/or heartache. There's also a distinctly different "golden flower" involved in the backstory — the buttercups that the Fallen Child poisoned themselves with.
    • Lastly, there's a smiley face, of all things. Associated with Flowey, the Amalgamates and the Fallen Child. While the symbolism isn't exactly clear, all the things associated with it are demonstrations of what happens when Determination goes horribly wrong.
  • Arc Words:
    • The word "Determination" comes up several times throughout the story in relation to the protagonist as well as humanity. Over the course of the game, the player comes to learn exactly what Determination is, how it makes humans and monsters different, and what the different parties involved in the story want from it.
    • "But nobody came." The phrase not only appears in a couple specific instances of the Pacifist/Neutral route (such as when you fight the three Memoryheads), but seeing that phrase in the encounter screen is a sign that you could be in a No Mercy runnote . If you make a call immediately after Toriel dies, you will get this as a message instead. It also crops up when Flowey talks about when he first woke up as a flower.
      Flowey: "Mom! Dad! Somebody help me!" I called out.
      Flowey: But nobody came.
    • "It's rude to talk about someone who's listening" has popped up a few times in relation to Gaster.
    • "Don't you have anything better to do?" It's mostly just a catchphrase for Flowey, but it's present in a few other places as well, and ties in with one of the game's major themes.
    • Most of the characters in the game have at least one moment where they say the sentence "Here we are." The two noteworthy things about this phrase is that it's the title of track 83 of the game's soundtrack, the theme of the True Lab, and it's the text that appears when the player uses the Check action on the Real Knife in the No Mercy route.
    • "Hopes and dreams" comes up a lot. Undyne uses this phrase multiple times in all variations of her pre-battle monologue (except one) and during a neutral/pacifist battle. "Hopes and Dreams" is also the title of Asriel's first battle theme.
    • "I can feel everyone's hearts beating as one" gets repeated a lot in certain battles.
    • "Humans. Monsters. Everyone." comes up in a few places, most notably in the boss fights against the final boss in a Neutral run and Undyne the Undying in a No Mercy run.
  • Arrange Mode: In the Hard Mode, activated by naming the Fallen Human "Frisk", several enemies in the Ruins are replaced either by exclusive enemies, or enemies that would appear in the CORE outside of Hard Modenote . The end of Toriel's boss fight is interrupted by the Annoying Dog, and the game ends right there.
  • Ascended Fridge Horror: Toriel at the beginning prepares to destroy the entrance to Snowdin because every child that has fallen into the Ruins goes out, dies, and gets their SOUL harvested by Asgore. Before that, however, they spent enough time with her to outgrow their shoes, win her love, and evolve her into the My Beloved Smother that you encounter.
  • Ascended Meme: The rumors that you could get a Real Knife in the demo become truth in the full version. On a No Mercy route, it'll replace the Worn Dagger in New Home.
  • Ash Face:
    • When Papyrus is zapped by his electricity maze, he turns black and smoky.
    • In a Pacifist run, when cooking with Undyne, you turn up the heat too high and the stove explodes, turning her face sooty for a moment.
  • Arson, Murder, and Jaywalking: At one point in Snowdin, Papyrus sets up a gauntlet of traps including such dangerous weapons as a cannon, a flamethrower, a spiked ball-and-chain, a spear, and... the Annoying Dog dangling from a rope.
  • Ass-Kicking Pose: Implied by one of the ACT menu options during the Mettaton fight. If you do it while low on HP, your audience finds it even more dramatic.
  • Autobots, Rock Out!: The final boss themes from the Pacifist and Genocide routes, "Hopes and Dreams"/"Save The World" and "Megalovania", heavily feature the use of electric guitars in their composition. The latter piece can be seen as playing with the trope, since it only plays when Sans fights you in the Genocide route; he's the one rocking out against you!
  • Award-Bait Song: The track "Undertale", which is played when the player is told the story of Asriel and the first human. Despite being an instrumental, it contains multiple criteria of an award bait song; it starts off mellow, gradually builds up, contains a modulation in the middle, and is used in a Tear Jerker context. Averted, though, mostly by the fact that it's not that different than the rest of the music and is crucial to giving that scene so much meaning and emotion rather than just being thrown in to get an award.
  • Awesome, but Impractical:
    • Most weapons are useless on a Pacifist run, as you won't be attacking anything. The only weapons that will be of any use are the Torn Notebook (which increases Mercy Invincibility), the Burnt Pan (which increases the power of healing items, but isn't bought), and the Stick (which can be used as an item to instantly pacify dogs). There's also the Worn Dagger, which you may want on a Neutral-Pacifist run to fend off Asgore as well as possible, although even the Burnt Pan and especially the Empty Gun are technically stronger if the player is good at timing critical hits correctly.
    • Burgerpants sells some of the best healing items in the game... which are unnecessary in a Pacifist run, where you can only have 20 HP, and they are pretty expensive. For the record, you can get foods that heal more than 20 HP in Snowdin's store, far earlier for far less. However, in a Pacifist run, the Mettaton EX fight encourages you to buy some of them, as eating them boosts your ratings and allows you to more efficiently spare him.

    B 
  • Background Music Override:
    • In the Core, its theme music continues even during monster encounters. It's a very clever way of establishing that these monsters aren't like the rest – they're hired mercenaries, who aren't random citizens merely expressing themselves with magic, but are scripted encounters that are actively trying to kill you. As such, they're part of the area's obstacles and puzzles rather than monsters with their own random encounter theme.
    • In the True Pacifist ending, "Reunited" continually plays as you revisit all the Underground's previous areas and locations for the last time (except for Papyrus's room, for some reason). If you then enter the mystery room in Snowdin Forest, it switches to that room's theme.
    • If you play the spook music, it will still be audible in the area south of Napstablook's house. If you hang around long enough for an encounter, the monsters will stop and say, "What... is that music?" and will be too spooked to continue the encounter. Aaron is particularly affected by it, and his "Where Are They Now?" Epilogue in the end credits becomes "Paranormal Investigator."
    • On the Pacifist and most Neutral routes, you can eat dinner with Sans at the MTT Resort's restaurant, where "It's Raining Somewhere Else" (a slower, more melancholy arrangement of "sans.") will play. The music will replace the hotel's music until the player leaves the resort.
    • On the No Mercy route, once you've exhausted a particular region of Random Encounters and get into a "But nobody came" encounter, the music will continue even as you exit the battle interface. This will also happen if you get this message in a Neutral run that does not turn into a No Mercy run (e.g. clearing Waterfall's encounters after sparing Toriel and/or Papyrus), but the normal music will return at an appropriate screen transition.
  • Badass on Paper: Inverted by Sans. He is said to be the easiest enemy, with 1 ATK, 1 DEF, and 1 HP, but becomes That One Boss in the "No Mercy" route, with a big dose of Combat Pragmatism and Nonchalant Dodge, which means that the 1 ATK is 1 for every frame you're hit and the 1 DEF and 1 HP means nothing because you can't actually hit him.
  • Bad-Guy Bar: In Snowdin Town's local restaurant, Grillby's, you can find all 5 of the dog mini-bosses you encountered along the way (assuming they're alive, of course). Subverted in that they don't treat you like an enemy anymore and talk to you normally, which makes sense given that Word of God says most monsters aren't actually trying to kill you.
  • The Bad Guy Wins: If a No Mercy run is complete, the First Child takes control of the protagonist, who then proceeds to destroy the rest of the world. What's even worse is that this permanently affects the game by adding a No Mercy flag in your app data, and the First Child wins forever even after getting the True Pacifist ending. You can potentially get around this by going into the game's app data and deleting your save, but the Steam version of Undertale saves that app data in the Steam Cloud and therefore you are literally stuck forever unless you know how to clear that out.
  • Bait-and-Switch: The entire game is full of these to the point where the entire game can be considered one giant bait and switch. Feel free to add as many as you like to this page.
    • During the boss battle with Papyrus, he tells you to prepare for his "blue attack." At first, it seems just like all the turquoise attacks dealt with previously, in which you simply have to not move, but it turns out the attack is actually to turn your heart blue, subjecting it to gravity.
      Papyrus: YOU'RE BLUE NOW. THAT'S MY ATTACK!
    • The first attack from Undyne the Undying starts off very similar to the versions used in the other routes, being just a slow line of bullets coming from a single direction... and then it breaks into a much faster flurry of bullets coming from all other directions.
  • Bait-and-Switch Comment: Many in the cast's speech, mostly from Papyrus.
    Papyrus: HUMAN. ALLOW ME TO TELL YOU ABOUT SOME COMPLEX FEELINGS. FEELINGS LIKE... THE JOY OF FINDING ANOTHER PASTA LOVER. THE ADMIRATION FOR ANOTHER'S PUZZLE-SOLVING SKILLS. THE DESIRE TO HAVE A COOL, SMART PERSON THINK YOU ARE COOL. THESE FEELINGS...THEY MUST BE WHAT YOU ARE FEELING RIGHT NOW!!!

    Papyrus: [at the Dogi's station] THE STATION OF THE MARRIED DOGS...HMMM. DO YOU EVER THINK ABOUT DOING THAT SOMEDAY? MARRYING A DOG? [...] NAH...THAT'S WEIRD. THERE ARE WAY BETTER ANIMALS TO MARRY. LIKE SKELETONS!!!

    Undyne: [after Papyrus jumps out her window, shattering the glass] I can't believe he leapt through the window like that. Normally he NAILS the landing.

    Papyrus: GARBAGE, HUH? BOY, DO I KNOW GARBAGE!! AFTER ALL, I'M HOUSEMATES WITH A LAZY BAG OF TRASH! HIS NAME'S TRASHY. HE LIVES IN THE GARBAGE CAN. [...] YOU DIDN'T THINK I DIDN'T NAME MY GARBAGE, DID YOU?

    Muffet: [on a No Mercy route] [Alphys] even left a route for me to escape from~ She said she would block off the rest of Hotland for me~ Foolish nerd~ A spider NEVER leaves her web~ (Except to sell pastries~) Ah, but I do feel a little regret over it now...yes, I should have wrapped her up when I had the chance~ She looked like she would have made a juicy donut~~
  • Barefoot Cartoon Animal: Several characters including Toriel and Alphys, as well as many common monsters and some mini-bosses. Which makes one wonder why Toriel needs a sock drawer. Also on this list are Toriel's husband Asgore and their son Asriel.
  • Batman Gambit:
    • Papyrus pulls a pretty smart one on Undyne in order to get her to befriend the human.
    • Flowey pulls one on you, the player, if you get the neutral ending and spare him. In order to get everyone in the underground into one place and absorb their SOULs, he goads you into going down the Pacifist route, because he knows that you want to "win" and get the "best ending." Do what Flowey predicted, and you'll face Asriel.
    • If you do a second No Mercy run, the Fallen Child's speech will change, suggesting you try something different next time. Because now that they have your SOUL, completing the True Pacifist path will let them out into the world to presumably wreak havoc without your influence.
  • Be Careful What You Wish For: Burgerpants says this word for word, after admitting that he initially came to Hotland because working with Mettaton was his greatest dream. Now that he does work for Mettaton, he realizes he's a Mean Boss that somehow coasts entirely on Popularity Power, despite the MTT Resort being, in Burgerpants's words, "a labyrinth of bad choices".
  • Become a Real Boy: The Mad Dummy is an angry, non-corporeal ghost inhabiting a training dummy who wishes to fuse with its current body and become a store mannequin. In a Neutral/Pacifist run, Mad Dummy, being non-corporeal, is invulnerable, but gets driven off by Napstablook's tears. In a No Mercy run, the hatred it has for you fuses it with its body, sparing you immediately while also opening it up to damage. On the Nintendo Switch, in a sealed room under Sans' and Papyrus' sink, you can find a life-size Mew Mew doll which is now inhabited by the same ghost from the Mad Dummy who attacks you to try and fuse with her new body through The Power of Hate, but when that doesn't work you can talk to her, which begins to fuse her with her body through The Power of Love.
  • Begin with a Finisher: The final boss of the Genocide route, Sans, opens his fight by unleashing a huge barrage that's extreme even by the game's Bullet Hell standards. This is even more surprising since he also gets the first move in the fight, unlike every other enemy in the game.
    "Huh. Always wondered why people never use their strongest attack first."
  • Behind the Black:
    • The "conveniently-shaped lamp" the player hides behind in the first scene with Papyrus only covers the player from the camera's point of view; given that Papyrus was standing to the side, he should have easily been able to spot where the player was standing.
    • There's a crystal formation in Waterfall that the protagonist finds particularly beautiful, but it's on the southern wall. The only hint of this formation existing is calling Undyne and Papyrus in the area.
    • This is a core feature in the perspective puzzle in the Ruins, in which coloured switches are hidden from the player by pillars, but would be perfectly visible to the character.
  • Being Evil Sucks: The No Mercy/Genocide route seems designed to make you feel this. It strips away the majority of the charm of the game, having much less in the way of puzzles and character interaction, in return offering only an endless slog of killing every enemy you encounter in only one or two hits, alongside many a self-inflicted Player Punch. The only thing it offers to compensate is the two toughest boss battles in the game.
  • Better Than a Bare Bulb: If there is a JRPG or general video game cliché present in the game, it will be commented on. Either it'll be for a quick gag, or it'll become a major Deconstruction that the game ends up revolving around.
  • Big Bad:
    • Asgore Dreemurr, king of the monsters. Throughout the game it's made quite clear that besides the barrier, he is the only thing between you and your escape from the Underground. Downplayed when you learn that Asgore doesn't actually want to hurt you, but feels as though he has no choice in order to keep a promise he made in a moment of anger and grief when both of his children died on the same day.
    • Despite that, Flowey serves as the most clear antagonist by both coming across the player character several times and knowing of the save file and using it to manipulate the player.
    • In the No Mercy/Genocide route, you. If you manage to complete it, the heroes lose. Irrevocably. Forever.
  • Big, Stupid Doodoo-Head: Sans's Trust Passwords are "I'm a stupid doodoo butt" and "I'm the legendary fartmaster." Of course, he asks Frisk to tell him the passwords.
  • Biting-the-Hand Humor: One of the shopkeepers mentions that "(If) you're really hurtin' for cash, then maybe you can do some crowd funding. I hear people will pay for ANYTHING nowadays." Undertale was, naturally, funded via crowdfunding.
  • Bittersweet Ending:
    • The best neutral endings, where you spare all the bosses, but don't complete all of the side quests, feature the Underground doing better with Toriel resuming her role as queen, but they have lost hope of ever returning to the surface. As a bonus, Undyne doesn't blame you for Asgore's death.
    • Even the best ending has some bittersweet, since without a soul of his own, Asriel will revert back into Flowey. He chooses to stay in the Underground, knowing full well he'll never see his parents again, because he doesn't want to keep torturing everyone.
  • Blamed For Being Railroaded: Muffet starts a fight with you because she sees you as a cheapskate for not buying any of her ludicrously expensive goods. This is despite the fact that, barring cheating or spending hours on grinding, there is no conceivable way you could have the money necessary to buy them. Less bad than usual for this trope, since the game portrays the price tag (and by extension, Muffet) as unreasonable to begin with. Then again, you had the option to buy spider goods for very cheap in the Ruins, so if you have one of those...
  • Blatant Lies:
    • While the rest of it is technically true, the in-battle description for Sans also calls him "The easiest enemy". Said boss opens up with a wave of hard-to-avoid attacks, making the lie seem obvious.
    • During a True Pacifist ending, Toriel pretends to be Sans in one of her texts to get back at him for writing down something she didn't say. Her use of polite language, proper capitalization, and the word "greetings" gives her away even before Sans manages to wrestle the phone away from her.
      • Similarly, Sans's "imitation" of Toriel is obviously not her, either. He's given away by the lack of capitalization and spelling.
    • In the True Lab: When checking a refrigerator that shakes intermittently, its description is:
      * (It's a refrigerator.)
      * (It's empty.)
  • Bleak Level:
    • The True Lab. Its only residents are Alphys and a mysterious new class of enemies, the color scheme is dull and dark, and it provides exposition about the Alphys's first experiments, Flowey’s origins, and the past of the Dreemurr family, all of which are… not lighthearted.
    • The entire game becomes progressively bleaker and bleaker on a No Mercy run, and it's especially noticeable if you've done a less murderous run earlier. The point when it really begins to show is Snowdin Town; the normal run town is the most populated area in the game, but it's almost entirely abandoned in a No Mercy run (apart from Monster Kid), and most of the flavor text has been changed to be much darker.
  • The Blind Leading the Blind: Undyne admits, after giving a violent cooking lesson that leaves her kitchen a mess and her house in flames, that she now understands why Papyrus, her original student, sucks at cooking.
  • Blood from the Mouth:
    • Seen only on a No Mercy run after dealing the lethal blow to Sans, though it might actually be ketchup.
    • If you kill Toriel in a genocide run or when her guard is down, a dark line is seen running from her mouth to her chin. It's still monochrome unlike the above example, so it's not as clear exactly what it's meant to be, but the imagery is still there. The worse option, of course, is that that's the cut from your 'toy' knife.
  • Bloodless Carnage: Monsters turn to dust when killed, so no blood is spilled. Subverted (or Averted, depending on player interpretation) once in the No Mercy run, as mentioned above. There is some evidence to suggest that monsters shed blood when injured prior to turning to dust, however, as also noted above, but since the only example we see is from a Boss Monster — a very powerful kind of monster with unique biology even by monster standards — nothing can be said for sure.
  • A Bloody Mess: Many fans have theorized that the red substance that begins to leak from Sans after the player delivers the coup de grace is just ketchup. Since Sans is shown to drink ketchup on other routes when he takes you to Grillby's, it's not that much of a stretch.
  • Blunt "Yes": If you complete the No Mercy path and destroy the world, a certain character will suggest that you think you're above consequences. You get a yes/no prompt to this — if you answer "yes", not only are you engaging in this trope, but they'll give you one right back.
  • Body Horror: Shows up very late in the game.
    • Photoshop Flowey — equipped with four eyes, vertically aligned teeth, a TV screen that alternates between showing Flowey's own dementedly-grinning face and a horribly distorted human head, and pipes that resemble intestines coming out of his body.
    • Each of the Amalgamates in the True Lab is multiple monsters fused together into a single wretched abomination. They're the end result of Dr. Alphys's attempts to inject monsters with Determination gone horribly wrong.
    • An earlier example that foreshadows the Amalgamates: If you kill Undyne in a Neutral run, she keeps going for a while, but her body slowly distorts, eventually melting before finally turning to dust. Naturally, she has more Determination than any other monster; this same Determination combined with a lack of doubt about the player's motivations is what causes her to become Undyne the Undying in the Genocide run. The transformation is unsuccessful in a Neutral run; likely, this is because she's not completely sure that the player is actually and wholly evil.
    • Flowey turns out to be Asriel reincarnated into the soulless body of a flower, also caused by Alphys.
    • Some lighter examples: Some of Flowey's expressions are pretty disturbing. (Spoiler warning for the link!)
  • Bonus Boss: In the Switch version, the Dog Shrine added in the PlayStation versions is replaced with one. Near the end of the game, you can unlock a hidden red and blue door behind the Dog Shrine with the analog sticks. This leads to a hidden room containing Mad Mew Mew, a life-size statue of the titular protagonist of the "Mew Mew Kissy Cutie" series possessed by Mad Dummy.
  • Bookends:
    • Both the first level and the Neutral endings conclude with you walking into a doorway, leading to a fade to white before cutting right to the game's title.
    • The last room of the first level is identical in appearance to the first one, with the same character commenting on the choices you've made since you met them at the start of the game. In the demo, this was the last room in the game. A third copy of it is the last room in the final game, too.
    • At the beginning of the game, Toriel takes you to her house in the Ruins. It's a quaint little house with three bedrooms, one of which is locked off. At the end of the game, you reach Asgore's domain, an identical house called New Home. It also has three bedrooms; the room that was Toriel's is locked, and the room in Toriel's house that was locked is Asgore's. This visually demonstrates the relationship between Asgore, Toriel, and their children.
    • Near the end of the Pacifist run, Toriel knocks Asgore away in the same way she knocked away Flowey in the beginning of the game. She even gives the same line of dialogue about a miserable creature torturing an innocent youth, which tells you exactly how she feels about her ex-husband...
    • Both at the start of the game and in two different endings, Frisk is saved from an unavoidable-ring-of-bullets attack by the last-second intervention of a third party.
    • If you backtrack to the beginning after sparing Toriel, she tells you "Don't worry about me... Someone has to take care of these flowers." If you backtrack to the beginning after sparing Asriel, he tells you the same. The reason for this is that the First Child's dead body is buried under these flowers.
    • The very last chords of "Bring it In, Guys", the theme for the ending credits, are the very first chords from "Once Upon a Time", the first song you hear upon starting the game. Subverted when it gets interrupted and you get more credits with a different song, and then Double Subverted when the ending of that song is another reprise of Once Upon a Time on the piano.
    • You can pull an I Surrender, Suckers on the first boss, dealing a One-Hit Kill. On a No Mercy run, the final boss can pull the same trick on you, to the same effect.
    • The first formal encounter, Froggit, is the last enemy to see you off at New Home. He's also the first one to tell Asriel's tale.
    • The end of the pacifist True Final Boss battle is a callback to the end of the Toriel battle, the first major boss. It reuses that boss's final "attack," and then there are several rounds of you selecting the same command while the boss talks instead of attacking you.
    • The first weapon you can find is a Toy Knife. On a Genocide Run, the last weapon you can get is the fabled Real Knife.note 
    • The first time you meet Sans is on a long straight silent trail where he initially uses the huge trees to remain unseen, and he's completely shaded over until he reveals his identity. The last time you meet him on a neutral or genocide run is in a long straight silent hall where he initially uses huge pillars to remain unseen, and he's completely shaded over until he reveals his identity.
    • When you first go to sleep in Toriel's house, you will find a slice of pie on the floor upon waking up. In the Pacifist ending, if you decide to stay with Toriel, The Stinger will show her sneaking into your bedroom at night, leaving a slice of pie on the floor.
  • Bootstrapped Leitmotif:
    • The Ruins theme is a lot more commonly associated with Undyne than its original context. Rearranged versions are also used for the general Waterfall and Hotland themes and a few cutscenes, with Snowden being the only notable exception.
    • "sans." tends to show up whenever something humorous or lighthearted occurs, like during the prelude to Papyrus's 'date' and the Blook family farm scene.
    • "Dogsong" is generally associated with dog monsters, namely Greater Dog and the Annoying Dog, but considering the latter's attitude and the song's use after Sans pulls his I Surrender, Suckers, it's also gained association with trolling-related moments.
  • Boring, but Practical:
    • In a Pacifist run, the healing items you can get in early areas like Snowdin can fully heal your lower health bar, and are cheaper than some of the fancier stuff later on. Bisicles are especially nice, since they have two uses, which helps bypass your limited inventory.
    • On the No Mercy run, you'll be one or two-shotting literally everything anyway, with one exception, and the Real Knife is functionally pointless by the time you reach it. The only weapon that's particularly worth your while after Undyne the Undying is the humble Burnt Pan, since it grants a slight bonus to HP gained from food, and you'll need a lot of food for fighting Sans.
    • The Burnt Pan is also more useful than later weapons for Pacifist and True Pacifist runs for the same reason, since you won't be using it to attack, anyway. It, along with the Stick and Torn Notebook, are the only weapons that have a use for something other than fighting.
  • Born of Magic: Monsters are described as having bodies mostly made of magic, as opposed to humans', which are mostly made of water. Descriptions of how monsters have children is not explicitly explained, but it is implied that the process is more magical than biological. Given the previously stated information, that may make little difference from a superficial point of view, but who knows.
  • Boss Banter: Many bosses are fond of talking to you during their battle (as are regular enemies, for that matter), but the final bosses of each of the routes fit best. The neutral one gloats about killing you and your friends, while the other bosses sound more like they're ranting on their personal soapboxes than trying to kill you.
  • Boss Remix: Several bosses have remixed versions of their overworld theme as their boss theme.
    • "Bonetrousle" is a remix of Papyrus's "Nyeh Heh Heh!"
    • "Dummy!" is a remix of a previous boss fight theme, "Ghost Fight". "Spider Dance" has bits of "Ghost Fight" in it as well, since Muffet is only fighting you due to Mettaton's interference.
    • "Spear of Justice" is a remix of Undyne's "NGAHHH!" Also, faster-paced remixes of "Ruins" and "Waterfall" are in "Spear of Justice".
    • "Death By Glamour" is a remix of "The Core" combined with Mettaton's "Metal Crusher" and "It's Showtime".
    • "ASGORE" is a remix of "Determination", Asgore's "Bergentrückung", and Toriel's "Heartache."
    • "Your Best Nightmare" contains Flowey's "Your Best Friend" during the SOUL attacks. It's replaced with "Finale" once the tide turns in your favor, which is also a remix of "Your Best Friend", and has a remixed "Memory" in the background during parts of the song.
    • "Hopes and Dreams" and "SAVE The World" both contain the main Undertale theme, "Your Best Friend", and "Memory", which turns out to be Asriel's own theme.
    • Most boss themes (excluding that of Snowden) are remixes of their respective area's overworld theme. They're all based on the same theme yet are all so extremely distinct as well as perfectly aligned with the tone, mood, and characters that they represent.
    • A track titled "Song That Might Play When You Fight Sans" is a remix of Sans's theme, "sans." with a few bars from "Bonetrousle" and, if one listens carefully, Gaster's Theme. It was left out in favor of "MeGaLoVania."
    • Inverted with "Dogsong", which is played during the fight with the Greater Dog — although it isn't particularly ominous to begin with and it's actually a sped-up remix of the normal battle theme. It's slowed down and played with (somehow) sillier instruments to become the theme for the Tem Shop in Temmie Village.
  • Boss Rush: Downplayed toward the end of the True Pacifist route. During the battle with Asriel, you must "save" the SOULs of the major cast, which means refighting them for three or four turns each.
  • Botanical Abomination: At the end of the Neutral route, Flowey the Flower gains the power of the human souls and becomes an enormous, omnipotent beast called Omega Flowey or Photoshop Flowey. This form combines plant-like features with machine parts.
  • Bragging Rights Reward:
    • The Real Knife and The Locket grant stat boosts of 99 to ATK and DEF, but you'll have no real use for them because by that point of the No Mercy path, the only enemy left renders your stats pointless. This ties with the game's themes of guilting the player for their destructive actions, as they could very well drive things that far just for the sake of power growth and the satisfaction of seeing the Real Knife fan rumor become truth.
    • Getting to the LV cap of 20 requires killing every single random encounter and every boss. By the time you're at 20, there's no more enemies where you can benefit from it, as the last boss you encounter is killed automatically.
  • Brainwashed and Crazy: In the final battle of the Pacifist route, Asriel captures the SOULs of the six main NPCs (Toriel, Sans, Papyrus, Undyne, Alphys, and Asgore) and erases all their memories associated with you. This makes them attack you blindly, and it's up to you to bring them back.
  • Bread, Eggs, Milk, Squick: "Spider Bakesale down and to the right. Come eat food made by spiders, for spiders, of spiders!"
  • Breaking the Fourth Wall:
    • Characters vaguely remember you on subsequent playthroughs and may comment on how things transpired the previous time. The player can even reference having died before against certain bosses or having killed certain bosses on previous runs.
    • Getting the No Mercy ending again after getting it the first time will result in a massive session of Leaning on the Fourth Wall. The First Child discusses the nature of leveling up in RPGs and getting stronger through murder. The game very heavily implies that the First Child is a representation of the player to some degree: an absurdly strong, invincible entity that is summoned into a game as your pawn, allowing you to reset the world and mess around with its inhabitants to your liking with the ability to perform resets (read: New Game) and endless retries (read: Saving and loading).
    • The Neutral ending you'd get if you somehow did not meet any of the requirements for the other endings involves Sans directly calling the player just to tell them to file a bug report (unless you've hacked the game to deliberately trigger said ending).
  • Brick Joke:
    • During the Golden Ending, Toriel will remark on whether you flirted with her, called her "mother", or both (as well as which order you did them in, if you did both).
    • Speaking of, checking out Papyrus's racecar bed during the "date" has him mention that he'd like to drive down a long highway if he ever reaches the surface. He does just that in the best ending.
    • At one point within Sans and Papyrus's puzzle hijinks, Papyrus tells Sans he's frustrated about Sans napping all night, at which, Sans corrects him by saying, "i think that's called... sleeping." Later on, a certain phone call with Papyrus and Undyne involves Papyrus saying "LATELY [SANS]'S BEEN NAPPING OVER 7 HOURS A NIGHT...", which the latter then responds with, "... wait, isn't that just called sleeping?" (This phone call also reveals that Papyrus himself rarely sleeps, in spite of having a bed himself.)
    • One of Papyrus's puzzles, designed by Dr. Alphys, has a ludicrously complicated set of rules... complicated enough that even Papyrus can't keep track of them due to her poor handwriting, and if you say that you didn't understand the rules, he tries repeating them, but gets them mixed up and gives up on the puzzle entirely. About two and a half areas later, you're asked to remember them all, and the only practice you could have gotten was on an... underwhelming "randomly-generated" puzzle. Thankfully, you're not expected to succeed in finishing it within the given time limit, although it's still possible.
    • Near the beginning of the game, the player encounters a spider bake sale. Players can put money in a spiderweb and a spider will bring them a donut or some cider. It's a cute and original way to obtain healing items before encountering the first town. A few areas later, the player meets more spiders who are hosting a bake sale, who are none too pleased if the player has not supported their enterprise. The player will have to survive an encounter with Muffet, their leader. If the player has bought a donut or cider earlier and consumes it during the encounter with Muffet, the "Spare" option will immediately become available.
      • Lampshadeed in the Genocide route, where Muffet will simply spare you because you haven't killed any spiders up to that point. Read: There are no spider characters besides Muffet.
    • During his boss battle, Papyrus boasts that "THE KING WILL TRIM A HEDGE IN THE SHAPE OF [HIS] SMILE!!!". In the pacifist ending, this is exactly what happens.
    • Also in one of his call queries, Papyrus wonders why they haven't come up with cool inventions like "PHONES THAT CAN TURN INTO JETPACKS". Guess what Alphys adds to your phone when she upgrades it?
    • At one point during Undyne's chase against you, Papyrus will call, stating you, he, and Undyne should get together sometime. Calling Papyrus within the same area after befriending Undyne will result in Undyne asking Papyrus what he called you about. Cue a bewildered reaction from Undyne.
    • Toriel has a scandalous sock drawer. Sans scandalously leaves his socks thrown about in a corner of his bedroom. If Shyren successfully sings, people will throw scandalous socks on the stage.
    • In the playable epilogue, Toriel's pie in her kitchen seems to have been eaten by dogs. At the end of "Hard Mode", you find it was the Annoying Dog who ate it.
    • Also in the epilogue, the mice will have finally gotten some cheese (or spaghetti). All four of them.
    • When first dealing with Papyrus, he has a trap about trying to make you eternally eat spaghetti (which would fail anyway since it's frozen). He will casually mention he might eat it himself afterwards. When walking back during the playable epilogue, you will find that Papyrus or the mouse indeed somehow had heated it up and eaten some of it.
    • On the way to Snowdin, you can encounter a Snowman who wants to see the world. If you carry a part of it with you through the final neutral boss and have not killed Papyrus, Sans will congratulate you. In the Golden Ending, if you return to the snowman with his piece, he will thank you, and ask you to keep holding onto it outside the underground.
    • The married dogs you meet in Snowdin mention placing second in the '98 Nose Nuzzle Championships. Asgore has the trophy for placing first. Also, if you complete a True Pacifist Ending and don't use the Stick to spare them, their yellow credit text says "Finally Nose Nuzzle Champions."
    • Monster Kid mentions how cool it would be for Undyne to show up at his school and beat up all the teachers, but resigns himself to the impossibility that Undyne would hurt anyone innocent. If the player calls Undyne while outside the school in Hotland, she excitedly offers to beat up the teachers as a way to make school more interesting. And if you call her again in the same place a second time, she will also say that it would be too mean. And in the Neutral-Pacifist ending where everyone but Asgore lives, she herself winds up becoming a teacher at Toriel's school.
    • On the Steam page for the game, it asks you to count the amount of dogs in the game. When you encounter Endogeny in the True Lab, the text says "It's unclear how many dogs this counts as."
    • If you examine the plants in Toriel's house after reading the book on the shelf in her room, the narration will identify the cattail plants as a "Water Sausage". Shortly after leaving the Ruins, you can examine Sans' first sentry post and find bottles of ketchup, mustard, and relish inside it. Way later in Hotland, we get a 2-for-1 payoff: Sans is actually selling Hot Dogs at his sentry station here... but if you buy one and examine it, you'll see that he just pranked you and it's a Water Sausage and not actual meat. However, the item will still heal you.
    • Early on, Toriel asks you if you've got any allergies. As it turns out, this concern is entirely justified, as the human is allergic to Temmies.
    • In Snowdin, you may encounter a large wolf-like monster taking huge blocks of ice off a conveyor belt and throwing them into the river. Later on, while being chased by Undyne, you'll see these same blocks flowing below. Finally, you'll see them dropping into the Core, ostensibly as cooling or for making steam.
      • When you return to Snowdin in the Playable Epilogue and talk to him, he's glad that he's finally able to take a break from his job and go buy some pants.
    • A while after you reach Snowdin during a genocide run, Sans will warn you that "if you keep going the way you are now... you're gonna have a bad time." Sure enough, if you get to Sans's boss fight and survive his first attack, the flavour text reads "You feel like you're going to have a bad time."
    • Examining a cactus in Toriel's house has the text call it "The most tsundere of plants". The only other cactus in the game is in Hotland, where the Tsunderplane can be encountered. It gets a second payoff in the Playable Epilogue on the Golden Ending.
      It's not like this cactus was waiting for you to come back, or anything...
    • If you decide to explore the upper floor of Alphys's lab, you'll find a machine that has what appears to be grass sticking out of the top, and a tube dripping pink goo into a bucket from one side of the machine. At the end of your "date" with Alphys, we learn what exactly that machine was and what its contents were; when she confesses everything to Undyne, the first confession is "I told you that seaweed was like...scientifically important...Really, I just...use it to make ice cream!" In a phone call with Undyne, it is also revealed that on several occasions, the only reason she didn't kill you on the spot was that Alphys had lied to her about the importance of the seaweed.
  • Break Them by Talking: In addition to being broken physically and psychologically, you're constantly on the receiving end of this during the final battle of the Genocide route. Theirs is a particularly penetrating example as Sans uses his greater scope of the world he lives in to speak to the player (whether it's directly or indirectly is up to interpretation), and he clearly understands what kind of person you must be if you made it this far without turning back.
  • Buffy Speak: Sans calls Papyrus's fence in Snowdin Forest a "gate thingy".
  • Bullet Hell:
    • Enemies attack with bullets of varying shapes and patterns. Battles with multiple enemies can become hectic.
    • Beware that some of the attacks will come from out of the square your heart is in.
  • Burger Fool: MTT-Brand Burger Emporium, even down to the mandatory slogans. Management is incompetent in several respects and outright sadistic in others, alternatingly micromanaging and operating entirely on whims. The leitmotif is the same pitched-down version of "Shop" you'd hear in other stores during a No Mercy run, no matter which end you go for.
  • ...But He Sounds Handsome: If you type "Toby" in the essay question during the Mettaton EX battle, he responds "Toby? What the hell is that? Sounds... sexy."
  • But Thou Must!:
    • Played straight a lot (all options during the "date" sequences will lead to the same outcome, along with many other places). But also parodied in many other places.
    • With the exception of the very first (whether you think Junior Jumble or crosswords are tougher), all of the many dialogue options with Papyrus will lead to the same result of him taking what you said/did as a compliment and liking you even more. This fits perfectly with his personality, as his ego, idealism, and naivety combined make him interpret you as a naturally nice person somehow complimenting him, even if you say things like "What a loser" to him (he thinks that you're calling yourself a loser, and then encourages you to think better of yourself).
    • When you're on the "date" with Alphys, she asks you how she can work up the gall to tell Undyne the truth. Your two response options are, "Let's Roleplay It," and "Obviously Let's Roleplay."
    • During Mettaton's quiz:
      Would you smooch a ghost?note 
      A) Heck Yeah
      B) Heck Yeah
      C) Heck Yeah
      D) Heck Yeah
    • During the Undyne date, when she asks you to pick something to drink, one of the options is labeled "Blatantly correct choice". No other option will actually advance the scene.
    • When you talk to him in the epilogue, Mettaton asks you for your opinion on a list of merchandise, but when he's done listing them, it just says "a yes or no prompt was not provided".

    C 
  • Call a Hit Point a "Smeerp": Even if it sounds like a lie at first, the game does use the term "LOVE" for LV instead of Level. Well, it does stand for Level Of ViolencE.
  • Call-Back:
    • In the Neutral run, when you first encounter Asgore, he greets you with a bit of small talk. In the No Mercy run, Sans repeats the conversation almost verbatim.
      Neutral Final Boss: Nice day today, huh? Birds are singing, flowers are blooming... Perfect weather for a game of catch.
      Genocide Final Boss: it's a beautiful day outside. birds are singing, flowers are blooming... on days like these, kids like you...
    • While walking with the Monster Kid in Waterfall, they tell you about how cool it would be if Undyne could visit their school and beat up all the teachers. They then take it back, saying Undyne would be too cool to beat up someone innocent. Later in Hotland, if you call Undyne in the room where kids are skipping school, Undyne comments on how she could make school a cooler place by visiting and beating up the teachers. She then takes it back, saying she wouldn't beat up a teacher.
    • If you run from the fight with Toriel in the Ruins and go to sleep, you'll hear a voice saying "Wake up, [player name]. You are the future of humans and monsters," essentially telling you that you have to go through with the Toriel fight. Towards the end of a true Pacifist run, in the True Lab, you can hear Asgore say the same thing in the VHS recordings, with a much darker meaning. That wasn't a guiding voice the first time you heard it. It was a memory.
    • Several bosses in the game (including Doggo and Papyrus) "block the way", but it's intentionally done with the first major boss in the game and the last boss of a True Pacifist run.
    • Going through the opening sequence multiple times will elicit different responses from Flowey. Eventually, he'll just say "Don't you have anything better to do?" After you exhaust all of Asriel's lines in the Playable Epilogue, he'll say the exact same thing, complete with a knowing smile (as he used to be Flowey, and may well have said the line earlier).
    • If you talk to Toriel before "fighting" the training dummy, she will suggest you start with a joke, and gives an example: "What does a skeleton tile his roof with? SHIN-gles!" In the true pacifist ending, she tells this same joke, much to Sans's delight and Papyrus's dismay.
    • Invoked during the "I Know You're in There Somewhere" Fight. By making the Lost Souls remember the good times they had with you (and their subsequent character development), you manage to SAVE them. Averted in the Final Boss fight in the Kill ’Em All route. Sans will mention that he senses something good inside you, but if you jump on it and do act or spare him, he will insta-kill you, all while mocking your naivety that you even believed for a second that he could spare his brother's murderer, though he also hopes he did get through to you, and asks you not to come back.
  • Cash Gate: Downplayed as it's not strictly necessary to win, but buying a (9999G) spider pastry from Muffet's bake sale will allow you to completely skip her boss fight in a Neutral or Pacifist run. Good luck actually collecting that much cash, though....
  • Cardboard Prison: Papyrus's attempt to use his and Sans's shed as an impromptu prison for the player is quite unimpressive, to say the least. His only measure for hindering escape attempts is placing a fence across the room, which has such large gaps between the bars that it can be circumvented by walking out between them, and the door to the shed turns out to be locked from the inside.
  • Cassandra Truth: When you first meet Sans and Papyrus, Sans tells you to hide behind "a conveniently shaped lamp" while he covers for you. The next scene involves Sans invoking this trope for laughs by telling Papyrus the lamp might help him find a human, much to Papyrus's frustration.
  • Central Theme:
  • Cerebus Retcon:
    • Using save points has the most innocuous things somehow fill the protagonist with determination. These kind of lines are later used in a more serious context for the various Final Bosses, and it's eventually revealed that determination is an actual plot element of the game, and it's shown how it's not always a good thing, in the form of horrifically mutated creatures who were injected with it.
    • Sans's laziness is also given a similar treatment. He's aware that you have been resetting the timeline. Knowing that everything he does will amount to nothing since you can undo it whenever you want, he finally just stopped caring. He only breaks his nonintervention to stop the player from completing a No Mercy file, the only outcome that can never be fully taken back.
    • Alphys' complete lack of any social skills and low self-esteem is played very comically when you're heading through Hotland, to the point where she posts a "picture of herself" that's really a frilly trash can like it's a point of pride. Then you find out in the Pacifist ending that her low self-esteem stems from failure after failure at trying to break the barrier, the consequences of which were the creation of Flowey and several monsters falling victim to Body Horror (whose families are missing them and ask her regularly for some kind of closure). She's also very realistically depressed and has contemplated suicide (and in almost all endings where Mettaton or Undyne die, goes through with it).
  • Cessation of Existence: For all the pointless death you can inflict on all the characters, there's still the tiniest grain of reassurance that, given the game's heavy focus on things like SOULs, they have some kind of afterlife to go to once it's all over. Except Flowey, who is sentient but has no SOUL to speak of. Near the end of a Genocide run, he tells you that he tried to commit suicide not long after waking for the first time, thinking that life without the ability to feel compassion or love wasn't a life worth living, but managed to stop himself via a Reset when he was suddenly overwhelmed by a terrifying realization: "If you don't have a SOUL, what happens when you...?" This is made even worse when you kill Flowey just two fights later in the same run. There's hope that, since his actual SOUL passed on a long time ago, he'll be able to join it once he dies, but otherwise the outcome looks pretty bleak.
  • Challenge Run: Fulfilling certain requirements throughout the game will give you unique text at the end of a Neutral Run when Sans calls you. Completing the hardest one (Never using healing items, keeping the bandage on the whole game, and remaining at LV 1) has him outright comment that you like to challenge yourself before asking you not to brag about it.
  • Chekhov's Gun:
    • An optional one. When exploring the ruins at the beginning, the player can find some spiders' bake sale and can buy some food from them. This food comes in handy when confronted by Muffet later on; using the food during the battle shows that you are a friend of spiders and ends the fight.
    • During their lunch date, Sans will tell the player that Papyrus claims a flower occasionally appears to him and tells him secrets and predictions. While Sans assumes that someone is playing a trick on Papyrus using an Echo Flower, the "flower" is, of course, Flowey. And who tells Papyrus to gather everyone in one place at the end of the Pacifist route so he can absorb all their SOULs...?
    • Similarly to the bake sale, if you keep the pie from the beginning of the game all the way to the Asgore boss fight, eating it will lower his stats, and during the confrontation with Asgore and Toriel as lost souls, eating the pie will help you SAVE them.
    • Another optional one: if you sing with Shyren until she departs of her own accord instead of you sparing her, when you encounter Knight Knight in the CORE, you can sing Shyren's song instead of a generic melody, making her fall asleep in two turns instead of four.
    • If you read the books in the library early in the game, they say that it would take the SOULs of almost every monster to equal the power of a single human SOUL. In the True Pacifist ending, Asriel is able to use the power of every monster SOUL to break the barrier with six human SOULs instead of seven.
      • The only exceptions seem to be Napstablook and the person who likes hearing people knocking on the door, because neither of them answered their door when the flash of light happened.
  • Chest Monster: The True-Pacifist-exclusive dungeon has an enemy encounter disguised as a Save Point. One hint that the not-save-point is a trap is that it's blocking a path.
  • Close-Knit Community: The Underground is this — everyone knows each other (with the exception of monsters residing in the Ruins, since the entrance sealed off from the rest of the Underground; only Napstablook can travel between them freely), and everyone cares deeply for each other. If you kill even one NPC, no matter how minor, people will notice and care. It's probably inevitable it turned out this way; most of the monsters' population was wiped out in the war, and the few who remain are all stuck in the same relatively small area for the foreseeable future. If that won't force you to get to know your neighbors...
  • Color-Coded for Your Convenience:
    • Some bosses can change the color of your Soul, which changes the way its movement works. Red is the default, and can move freely. Blue subjects your soul to gravity, dragging it to the ground and forcing you to jump to move vertically. Green prevents you from moving, but gives you a shield that you can point in different directions to block incoming attacks. Purple forces you to hop between horizontal lines for vertical movement, but still allows you to move back and forth along those lines. Yellow allows your soul to shoot projectiles. All of these changes disable the Flee option.
    • Enemy projectiles have colors as well. White projectiles are normal and do damage when touched. Light blue attacks won't hurt you as long as you aren't moving, while orange ones will only hurt you if you're staying still. Green attacks will heal you when you touch them and/or must be touched to Spare an enemy. Grey attacks (only so far used by ghosts/objects possessed by ghosts) do nothing at all. They're used to relay messages in a non-harmful manner. Red attacks, similar to grey attacks, are often used as a warning in order to allow you to know when an attack is coming.
  • Color Motif: Playing the Ball game reveals tidbits of the significant traits colors represent:
    • Red's trait is unknown, but it's assumed that it may be Determination. This is the color of the protagonist's SOUL, and it's also implied to have been the color of the First Human's/the Fallen Child's/Chara's SOUL as well.
    • Orange represents Bravery; accordingly, when faced with Orange attacks, you have to unflinchingly move straight through them in order to avoid taking damage. It's implied that the Human with the Orange SOUL wore the Tough Glove and the Manly Bandana and that they fell somewhere in Snowdin, just outside the Ruins, the tutorial area, probably after running into trouble one too many times.
    • Yellow represents Justice; tellingly, it's one of the colors that Sans' eye flashes during your fight with him at the end of the Genocide route. It's implied that the Human with the Yellow SOUL wore the Cowboy Hat and Empty Gun. They probably fell in Waterfall, not far from Justice-worshipping Undyne's house, after they ran out of bullets, since Bratty and Catty, who you can buy the Hat and Gun from, explain that they find their "wares" in Waterfall's garbage dump.
    • Green represents Kindness; if you see green-colored "bullets" in battle, then they will heal you instead of hurt you and they quite often advance the battle, pacifying your opponents and allowing you to spare them. It is also the color Undyne turns your SOUL during your fight with her; while this prevents you from running away, it also gives you a shield with which you can block her attacks. The Human with the Green SOUL is implied to have donned the Stained Apron and the Burnt Pan, the former which lets you regenerate 1 HP every other turn, and the other which increases the HP recovery of healing items with +4.
    • Teal represents Patience; you have to stand still in order to avoid taking damage from these attacks. It is also the second color that Sans' eye flashes during your fight with him, and say what you want about Sans, but he has been nothing if not patient with you up until now. The Human with the Teal SOUL embodied this trait to the point that they never even left the Ruins area before they died.
    • Blue represents Integrity; it weighs your SOUL down with gravity, turning the gameplay into a platformer instead of a Shoot 'em Up. The Human with the Blue SOUL is implied to have worn the Old Tutu and the Ballet Shoes and, to all appearances, died in Waterfall, where lots of enemies have attacks that are harder to dodge at the bottom of the screen.
    • Purple represents Perseverance; when your SOUL is turned purple, you're trapped on three lines that you have to jump between in order to avoid the enemy's attacks.
  • Comic Role Play: En route to the Golden Ending, the player will find themselves on a date with Alphys. She admits that she likes Undyne, but can't work up the nerve to tell her. The player suggests that they roleplay as practice, and then chooses between serious or goofy role-playing lines. Of course, it spirals out of control regardless of the player's choices.
  • Completion Mockery: As noted in Achievement Mockery, the trophies show disdain over the trophy system. Get all the trophies, and you will get the platinum trophy, which is named "Don't You Have Anything Better To Do?"
  • The Computer Is a Cheating Bastard:
    • An in-universe example. Flowey closes the game before you can stop him from using the human SOULs. When you boot the game up again, he has already changed the world to eliminate all other threats and replaced your file with his. He then literally shatters that file, before you can overwrite or delete it. At the beginning of the fight, he makes a quicksave. Throughout the fight, he makes quicksaves right before attacking so he can unexpectedly load them later, but loading them doesn't give you back any of your health. When you finally beat him, he loads his first save, thus undoing all of your progress. (Or so he thinks....)
    • Another in-universe example: Asgore gets the first turn and uses it to destroy the MERCY button.
    • Even worse, Sans gets the first turn, attacks you on the menus by exploiting the fact that your SOUL is the cursor, completely ignores mercy invulnerability, and dodges your attacks. He's so good at reading people's faces that he can tell if you've died to him before and he exploits this knowledge by interrupting his own dialogue at varying points to attack you.
  • Context-Sensitive Button: The ACT option in battles. When it is selected for an enemy, a set of options unique for them pops up, and you can choose any one of them. The effects they have include doing nothing, changing your stats or the enemy's stats, affecting their next attack, allowing them to be spared or making them leave the battle, and any number of miscellaneous effects depending on the enemy.
  • Continuing Is Painful: Averted for the most part, except during Papyrus' fight. Unlike most battles, where you die and are sent back to the last place you saved, it's completely impossible to die during Papyrus' fight, as he captures you and the battle ends once you hit 1 HP. While this might seem like a good thing, this means that after every fight, the healing items you used in the previous fight are still gone, and in subsequent tries, you'll have to waste money to buy more healing items.
  • Contrived Coincidence:
    • The conveniently-shaped lamp that Sans tells you to hide behind is shaped exactly like the protagonist seen from the side and hides them perfectly.
    • The color tile puzzle that Papyrus generates for you appears to be this, since the "random" configuration that appears is two straight rows of passable tiles surrounded by red tiles. "Appears" being the operative word, because it's very likely that Mettaton (who was used to create the puzzle) had control over the outcome and decided to mess with everyone by making it so simple.
  • Controllable Helplessness:
    • You can turn in place while webbed up by Muffet, and move around freely while Endogeny approaches, but you can't escape in either case.
    • During part of the pacifist True Final Boss, you are unable to select any menu option except Act->Struggle, which just says "Can't move your body." Attacks during this phase are almost impossible to dodge, but it also doesn't matter how many times you get hit.
    • At the end of Flowey's boss battle, he traps your SOUL in a circle of bullets so he can gloat a bit before trying to kill you. You can still move your SOUL around, but touching any of the bullets just teleports it back to the center. Do it enough, and Flowey will actually get annoyed and yell at you to cut it out.
  • Convection Schmonvection: Zigzagged. While Undyne's armor heats up significantly while crossing a bridge over lava, to the point where she passes out, you are perfectly fine in that same area and suffer no heat related issues, even when you make it hotter to appease a monster. Then again, Undyne is a fish monster wearing heavy steel armor. Your character is wearing a simple striped shirt. Also, the Royal Guardsmen apparently have "cooling dirt" on their armor, which you need to polish away in order to get one of them to succumb to the heat and take his armor off as well. Since Undyne is normally stationed in Waterfall, her armor would have no need for such enhancement.
  • "Could Have Avoided This!" Plot: Two in the True Pacifist Ending:
    • When Toriel confronts Asgore, she tells him that he could've stopped his quest at one human SOUL, crossed the barrier himself, and taken six more human SOULs. Since she mentioned it could've ended their imprisonment peacefully, it stands to reason that she meant SOULs from recently-deceased humans, since it's established that human SOULs persist after death.
    • If you choose to stay with Toriel, she invokes this, saying the player could've said that at the very beginning, and that would've been the end of it. But she adds she's glad they didn't, since now they are free.
  • Crapsack World: Played for Laughs if you play in Hard Mode. You access Hardcore Mode by naming the Fallen Child "Frisk". In this mode, Toriel makes you snail pie, you face the hardest monsters first, and after you defeat Toriel, the Annoying Dog tells you it ends here.
    Narration after taking a third piece of Monster Candy: In this hellish world, you can only take 3 pieces of candy...
  • Crazy-Prepared: Alphys makes several upgrades to your cell phone that come in handy later. Some of these upgrades — the jetpack, the bomb defusal program, Yellow Soul mode — come in handy for parts of what turns out to be a Worked Shoot, so this trope was invoked. Things become much more difficult for Alphys when Mettaton goes Off the Rails in the CORE, forcing her to adjust/improvise several times in order to keep the player safe.
  • Creative Closing Credits: The special thanks portion has you dodging the names of the 918 Kickstarter backers who pledged high enough for that reward. Dodge all of them, and you'll be able to access the Annoying Dog's room in Snowdin.
  • Creepy Jazz Music:
    • Downplayed with "Ghost Fight". The music itself fits this trope, being a swingy, but somewhat haunting tune. But what prevents it from being a straight example is the context in which it plays. It's the boss music for Napstablook, a Shrinking Violet ghost who is reluctant to fight — certainly not the type of character usually associated with this type of music.
    • And then played straight with "Dummy!", the Boss Remix that plays while fighting Mad Dummy.
  • Crime of Self-Defense:
    • A lot of the monsters the player encounters will attack and usually try to kill them. The player may think attacking them back and killing them is self-defense, but the game treats it as murder. However, a book in the library says that magic is how monsters express themselves, which implies that damage done to the human, with the exception of the bosses and mercenaries late in the game, is incidental.
    • If you make certain choices, Sans will say he understands that you might have been defending yourself, but then he asks you if a person with a special power has a responsibility to use it to help people. If you say yes, his eyes go dark and he asks why you killed his brother, then disappears.
  • Cringe Comedy: The date with Alphys on the Pacifist route. Between Alphys's complete lack of social skills, and the fact that she's obviously pining for Undyne, it's awkward from the word "go." But then you start to roleplay an interaction between Alphys and Undyne, and things get even worse. (Or better, since that's when the scene goes from amusing to hilarious.) And then Undyne overhears you.
  • Crippling Overspecialization: The monster race as a whole in terms of power. Their main distinction from humanity is their association with magic, which allows them to, among other things, absorb a human SOUL once the carrier has perished. If even one SOUL is absorbed, a monster becomes exceedingly powerful — the game's backstory implies that they could level an entire village with ease — and if they absorb seven, they become a being on the level of godhood. Without a SOUL to absorb, however, monsters are so outmatched by humanity that it only takes a single, particularly determined child to wipe them all out if the player so chooses it.
  • Critical Existence Failure:
    • Monsters don't show damage, but disintegrate into dust after their health is depleted. The only ones who don't drop dead after being defeated are Undyne (who was probably running on borrowed time, anyway) and King Asgore. This might catch a player off guard if they weren't intending to kill the first boss.
    • This is actually averted for most enemies. Unlike in most RPGs, where characters take more or less the same damage regardless of health, enemies in this game take more damage as their HP gets lower. This can screw over players who try to spare monsters by beating them to critical health.
    • This is played straight for the player, whose SOUL takes the same damage from an attack no matter how much HP they have left. It's also something exploited by the more Technical Pacifist characters who don't want to kill you, such as Toriel and Papyrus; they're willing to reduce your HP to a low level, but once you're that weakened, they'll either deliberately avoid landing their attacks (Toriel) or just detain you (Papyrus). Since you don't seem to suffer any physical injuries from attacks to your SOUL, it seems to be fine with them. Asgore, who also doesn't want to kill you but feels that he has to, exploits this in a much more somber fashion by always giving you a Last Chance Hit Point.
    • Asriel averted this in the backstory; he was mortally wounded in the human village, but made it back through the barrier and to the castle before succumbing.
    • Played with during the final battle of the Pacifist run. As you're reaching out to save Asriel's soul, he lashes out with a final attack that deals an incredible amount of unavoidable damage. The player's health drops: first to one, then to .90, then to .50, all the way down to a billionth of a single hit point. As the game says at the beginning, though: you only lose when you reach 0 HP. Through determination alone, Frisk prevents that from happening. An alternate explanation of this involves Asriel not wanting to kill you, and thus being unable to do lethal damage.
  • Critical Hit:
    • While it isn't a game mechanic per se, there is something like it present: In a No Mercy run, attacking almost any major character will cause you to deal an extremely high amount of damage in a single attack, almost instantly causing death. This is caused by them being caught unprepared and by your LOVE and Determination being especially potent on the most prominent characters.
    • This catches a lot of players by surprise even in a Pacifist run: when fighting Toriel, many players try to weaken her to the point where she can be shown Mercy. Once she's down below 1/3rd of her health, though, the next hit will eliminate her. Toby Fox clearly feeds off the tears of his audience.
  • Crosshair Aware:
    • The final soul in the Photoshop Flowey battle spouts three crosshairs at once before letting off a salvo of bullets. This also helps you see where the healing items will be.
    • Asriel's Shocker Breaker is basically a carpet bombing using rainbow lightning, and there are exclamation marks right where they will land.
  • Cruel and Unusual Death:
    • At the end of the No Mercy route, you get to butcher Flowey repeatedly into smaller and smaller pieces until he's totally gone. Considering most fights end with a clean kill, be it from massive damage or a sneak attack, this is easily the most brutal way to die in the entire game.
    • In the backstory, the first human dies of buttercup poisoning, an incredibly painful way to die. Symptoms include bloody diarrhea, excessive salivation, colic, and blistering of the intestines. This is presumably to free the monsters or to exact revenge on the humans for unknown reasons.
    • Most monsters turn to dust when they die. Not Undyne. She has a greater amount of Determination than most monsters, meaning that in a desperate situation, she can will herself to stay alive for just a bit longer. The downside to this is that monsters' bodies don't respond well to Determination, so when the player kills her, her will to live causes her to start melting. She's fully conscious during this and even tries to take you down with her, though at this point, her magic is so weak she can't really do anything to you.
  • Cruel Twist Ending: If you follow a No Mercy run with a Pacifist run. Everything seems to be normal... up until the last moment, in which it's revealed that the player character has been possessed by the Fallen Child and is then implied to destroy everyone and everything within the game, apart from themself and the player character.
  • Cue the Sun: The pacifist ending has Frisk leading their new friends outside of Mt. Ebott into one of these, and they comment on how beautiful it is. This is due to the fight with Asriel taking all night, given the encounter with Asgore happens during sunset.
  • Curb-Stomp Battle:
    • As you can learn throughout the game, the war that the humans won against the monsters was one of these. In fact, not a single human was killed during the war. This makes sense when you consider that Frisk, who is a mere child, can completely eradicate the monster population single-handedly, and it's likely that adult warriors and soldiers probably fought in the war itself. Combine that with the knowledge that a human's killing intent and prior killing history makes them even deadlier to monsters, and it's no shock that seasoned, experienced soldiers were able to almost effortlessly wipe out most of the world's monsters.
    • In a No Mercy run, almost every single fight will end with you obliterating the monster with a single strike. The game is intentionally balanced to be challenging to a Frisk at LV 1, at their absolute weakest. There are, however, two notable exceptions who will turn this trope on you: Undyne the Undying, and Sans.
  • Cutting the Knot: The second "X's to O's" puzzle is more difficult than than the other two and can be frustrating to figure out. However, you can just check out a nearby tree, which has a hidden switch that immediately solves the puzzle when you press the button.

    D 
  • Damn You, Muscle Memory!: In the last leg of the battle against Asriel, the "SAVE" button is always flashing, which can trick you into thinking it's selected when it's not — after every SOUL you save, the cursor defaults back to "Fight", making it easy to accidentally attack Asriel. Not helping is the fact that, just before you're able to save your friends, "ACT", which "SAVE" takes the place of, is all you can select, and "SAVE" is selected by default the first time the option unlocks. As a result, you might not even realize "FIGHT" can be selected until it's too late.
  • Dark Horse Victory: If you kill only all the "Boss" characters like Toriel, Papyrus, etc., but spare every other encounter, then the Annoying Dog somehow manages to become president of the Underground, and Sans comments that "Somehow, everyone seems happier this way".
  • Dark Is Not Evil:
    • Although even the monsters refer to themselves as "monsters", none that are actually encountered within the game are truly evil, Fantastic Racism aside. In fact, most Random Encounters just want you to do something with them before they're willing to leave you alone. Even the few monsters identified as "bullies" will allow you to spare them simply by being nice.
    • In general, this can count for locations as well. Be it Sans's room or the abandoned lab, they are creepy and have pitch black areas, but there is nothing really dangerous there.
  • Dark Reprise:
    • In the demo, if you kill Toriel, then return to her house, her theme plays in a hauntingly lowered pitch and tempo, reflecting her death.
    • Once you're on-track for the No Mercy ending, the same thing happens to every area theme in the game.
    • After depleting an area of its monsters, "Your Best Friend" plays, slowed down to the point it's oppressive, booming, and nearly unrecognizable.
    • A distorted, solemn version of the Snowdin theme plays while "fighting" one of the monsters Alyphys injected with Determination, who is really the mother of an enemy from that area.
    • The True Lab theme itself sets Alphys' theme to dark harmonies, to accompany her backstory.
    • Zigzagged with Flowey's theme, "Your Best Friend", which turns into "Your Best Nightmare", a manic, distorted, utterly insane version when Flowey becomes Photoshop Flowey. However, it's eventually replaced with "Finale", a more hopeful version, once the player is finally able to start getting the upper hand on Photoshop Flowey.
    • Due to very different tempo and instrumentation, it's not obvious, but "Battle Against a True Hero" (found when fighting Undyne in the No Mercy route) uses a melody which is very similar to "Spear of Justice" (Undyne's usual battle theme in Neutral and Pacifist Runs). The similarity is due to the fact that "Spear of Justice" showcases Undyne as a Hot-Blooded, bombastic warrior who is filled with undue enthusiasm about attacking you, while "Battle Against a True Hero" shows the same character in a more solemn, serious state as a heroine trying to hold out hope in the face of unspeakable evil that seriously threatens to plunge everything into death and despair.
    • The song that plays during her last stand before dying is also a sad rendition of part of Undyne's theme (really a common tune throughout the game, but it appears during the preceding fight). The full version of that song plays during the neutral ending as well.
  • Dating Sim: Papyrus's sidequest includes a style parody of them. Alphys's looks like it's going to be a call back to this, but it's a bait and switch.
  • Deadly Euphemism: Throughout your journey, you'll hear about monsters who have "fallen down". It's revealed through Alphys' entries in the True Lab, and in Snowdin library, that "fallen down" means "comatose and near death".
  • Deal with the Devil: If you want to play the game again after completing a genocide run, you need to make one of these with the Fallen Child, giving them your SOUL in exchange for recreating the world you destroyed. The consequences of this deal don't become apparent until you try completing the True Pacifist run and discover that the Golden Ending is no longer golden.
  • Death of a Child:
    • You can kill the teenage Snowdrake (and the game makes sure to call you out on it when you meet his father).
    • The Plot-Triggering Deaths are a tragic example: Asriel Dreemurr and the first Fallen Human were both little kids when they died.
  • Decon-Recon Switch:
    • For the Determinator trope: Determination is an actual physical force that allows things to come back to life. Injecting it into most monsters turns them into the Body Horror Amalgamates, but the Determination wielded by the player and Flowey gives them the ability to SAVE. Flowey and the Genocide run player have abused their ability to save to kill everyone, and Flowey in particular makes use of the ability to SAVE in his boss fight to kill you over and over again. And it isn't much better in a Neutral or Pacifist run, since Flowey is manipulating the player's Determination to get the Golden Ending in order to get the power that he wants. However, in the Pacifist Ending, it's ultimately the player's Determination to save everyone, including Asriel, that allows them to hang on until the end and Earn Your Happy Ending.
    • Most of the game is a long, hard examination of traditional JRPG tropes and cliches, but things can easily be switched from a deconstruction to a reconstruction by simply playing through the Pacifist route. Unless you sold your SOUL beforehand.
  • Deconstructor Fleet: Though not immediately apparent, the game is a Deconstruction of RPGs, both on a superficial level from sarcastic flavor text and characters (staying in the Snowed Inn, for example, has the innkeeper give your money back, mentioning you were only up there for two minutes) as well as the deeper plot. It also examines 100% Completion on a meta level. What if there were alternate ways of getting from Point A to Point B, and the person going through the motions happens to have the ability to go back and explore these routes after going through one? What if they remember going through the motions? What if someone else can remember what they did in a previous loop? If the world inside the game is real, what happens to it when the player reloads a save, or starts New Game+? It also takes a look at a number of gaming tropes, like the Determinator, Save Scumming, Turn-Based Combat, Level Grinding, and, most of all, Video Game Cruelty Potential. And, for the truly curious, Dummied Out.
  • Defeat Means Friendship: Defeating your enemies non-violently allows you to befriend them. Subverted if you go the violent route — that just causes you to cut a bloodynote  swath through them.
  • Deliberately Monochrome:
    • Monsters are always monochrome in battle, but are colored on the overworld. This is because all monster SOULs are grey. The one exception is Froggit, which is the same color in both the overworld and in battle.
    • The game's intro sequence is in sepia. This is to mask the fact that the human you see in the intro sequence is not the main character, but rather, the first human to have ever fallen into the Underground, revealed by a Wham Shot in the true ending.
    • New Home is a colorless shade of white and grey, the only color in sight being the golden flowers scattered around the house area. This is presumably meant to represent how lifeless Asgore's world has been since the death of his children.
  • Dem Bones: Sans and Papyrus are an interesting case: They look like human skeletons, yet they are actually a species of monster. There is no mention of them dying in the past, implying that they have always been skeletons. Supported by Papyrus thinking that humans "descended" from skeletons, showing that their skeletons are different.
  • Developers' Foresight: Has its own page.
  • Developer's Room:
    • One is hidden a few screens before Snowdin. You can unlock it by successfully avoiding the special thanks credits during the true pacifist ending. Sadly, you cannot fight the dog inside...
    • Two appear as placeholder screens in case something goes wrong within the game. They involve the Annoying Dog either standing within the center of the screen or sleeping while one of two otherwise unused songs (depending on the screen) plays in the background. The only way to get out is to reset the game.
  • Diagonal Speed Boost: The game doesn't reduce your horizontal or vertical velocity if you move diagonally. While this can be beneficial on the map, it can feel awkward in battle, particularly for players of Shoot Em Ups that are used to this trope being averted. Fortunately, it can be disabled in the options menu.
  • Die Laughing:
    • If you dispatch the first boss in a particularly cruel way, they'll declare that you're worse than the monsters. Either slay as many monsters as you can, or get the boss to back down and then attack them.
    • Some foes in Hotland (Muffet, Madjick, Astigmatism) make a giggling sound when you land a hit on them, which results in this trope if you One-Hit Kill them during a No Mercy route.
  • Difficulty Spike: The Pacifist and No Mercy routes are markedly harder than playing normally in Neutral. In Pacifist, you must avoid killing absolutely everything, which means you will not gain any attack power or HP and will have to get really good at dodging. In No Mercy, you have to kill absolutely everything to get super strong, which means lots of grinding to purge all random encounters. The few bosses that can pose a challenge are absolutely brutal; the game will get really easy as you gain EXP and get stronger, till you can one-shot most enemies, including bosses, but two bosses are still capable of giving you a good fight, as they're the toughest in the game.
  • Disc-One Nuke: You can get the Temmie Armor before you are even halfway through the game, though this requires an enormous amount of money. Not only does it have the highest defense value of any armor in the gamenote , it also increases your attack, the invincibility frames after getting hit, and restores one point of health every turn in combat.
  • Do Well, but Not Perfect: In Thundersnail, if you win, you earn 9 G, which is less than the entry fee of 10 G! Napstablook explains that they have to make a profit somehow. If you lose by a very narrow margin, Napstablook will instead give you 30 G to avoid disappointing the snail you were cheering on.
  • Do Wrong, Right: Toriel points out to Asgore before the Boss Fight with Asriel that instead of waiting for other monsters to kill seven humans and take their SOULs, he could have only killed one, crossed the barrier by combining the soul with his own, and gotten six more from the surface. He can't deny that she has a point.
  • Does This Remind You of Anything?: The first thing to emerge from the reveal of Mettaton EX is something crotch-level and long. It's probably the most explicit thing, in a fight loaded with innuendo.
  • Don't Make Me Destroy You: The final boss of the No Mercy route takes this attitude with the player character once they first meet. The boss says "you are really not gonna like what happens next" if you fight them, trying to warn the human not to advance. When the human steps forward anyway, the boss gives a glorified shrug and starts the battle.
  • Double Unlock: The Temmie Armor requires you to pay one thousand gold to get the option to buy it, then a very high amount of gold to actually obtain it.
  • Down in the Dumps: Partway through Waterfall, you fall into the Trash Zone, which, fittingly, is filled with garbage. And a mini-boss. You return there for Alphys’ date.
  • Downer Ending: Generally speaking, the more monsters you kill, the more of a downer the ending is. The worst ending, naturally, follows the genocide run and actually gets even worse if you follow the genocide run up by selling your SOUL and doing another pacifist run.
  • Dramatic Wind:
    • When Papyrus poses and speaks about how great he is, a breeze ruffles his cape.
    • The save point at Undyne's battle arena, plus a random line when fighting her, "The wind is howling."
    • Asgore's cape noticeably billows as he fights you.
    • When the cast all meet to convince you and Asgore not to fight, Undyne's sprite has her hair blowing, while nobody else is affected.
    • When the Fallen Child sends you to a black screen to wait for ten minutes, a sound like wind blowing can be heard in the background.
  • Dreamworks Face: One of the sprites in the game files shows Toriel making this face and is named spr_face_torieldreamworks_0. Papyrus also has a confident expression during the battle with him.
  • Driving Question: An interesting variant, as it is one not posed to the characters, but rather the audience: "Don't you have anything better to do?", referring to the need to 100% games some people feel when completing video games, and asking said player if they are willing to commit heinous acts to see everything the game has to offer.
  • Dude, Not Funny!: Invoked and seemingly parodied. In one battle, you have the option to tell Woshua, a Neat Freak monster, one "dirty" joke about "two kids who played in a muddy flower garden," another about "a kid who slept in the soil," and a final one about "a kid who ate a pie with their bare hands". Woshua doesn't react well to these jokes and reduces his attack. Then on a Pacifist run, you later learn about the king's children Asriel and the Fallen Child accidentally feeding their father a pie with buttercups instead of cups of butter, that they picked the flowers, and of the Fallen killing themselves with buttercups and wanting to be "buried in their village" when they wanted to kill all the humans once across the barrier. Think about it and you realize that you are joking about the last hope of monsters being killed, which makes Woshua's reaction more heartbreaking.
  • Due to the Dead: With monsters, this trope is a bit difficult to do since the monsters, when they die, turn into dust, and for this reason Alphys accidentally infused a flower with Asriel's remains, trapping him in that state. For humans, however, their bodies notably don't dissolve, so Asgore and Toriel show a lot of respect for the corpses even as Asgore takes their souls in preparation to break the barrier. Toriel also took the Fallen Child's body and buried it in a field of golden flowers, the field on which Frisk lands.
  • Dummied Out: Quite a bit of cut content can be found by hacking into the game:
    • There's battle text and sprites for a female dog guard named Doge and two female Royal Guard members based on a cat and a bug.
    • The mystery of Dr. W.D. Gaster is centered around a bunch of NPCs, music, and rooms that were initially only found through hacking, but with the 1.001 update, there's now a small chance that most of them can be encountered normally.
    • The protagonist has a set of sprites with Hidden Eyes and a green-yellow striped shirt. This might have been intended as their reflection after completing a murder run and selling their SOUL, but it only actually appears with a Debug Mode option on, and even then only in one room in Waterfall.
    • Parodied with a song on the Official Soundtrack titled "Song That Might Play When You Fight Sans". It doesn't show up in the finished game's files, thus it does not play when you fight Sans.
    • In the demo only, there's a couple instances of text and file names referring to a "Grandpa Semi". The inclusion of semi-serif fonts in the demo suggests that another skeleton character was planned at some point. The "grandpasemi" name was eventually renamed to "grandpatemi" and repurposed for the Temmie enemy, if you annoy one by denying her Tem Flakes.
    • There's a unused regal-sounding remix of the Game Over theme within the game files. According to Toby, this would have originally played when the player met Asgore instead of "Small Shock", but it didn't fit the mood of the scene.
    • A short, upbeat tune can be found within the game files. It only plays during a placeholder screen involving the Annoying Dog that only appears if something goes wrong. Similarly, a different song plays during a similar placeholder screen used for a similar purpose: a lullaby, fitting due to the Annoying Dog sleeping on that screen.
    • There are a few unused enemy formations, including Jerry by himself; presumably cut because it would be an annoying and pointless encounter. Though, given that it's Jerry, that might also be why it was considered in the first place.
    • The "dirty hacker" ending is an exploited example; if you play the game without hacking and see it anyway, something has gone horribly wrong.
    • Mettaton EX has responses for if you type "sexy", "foxy", or "tantalizing" in his essay question, but since the X and Z keys do nothing here for obscure technical reasons, you can't ever see them.
    • An accidental example: At the beginning of the game, a random "fun" value is written into your ini file. Throughout the game, certain Easter Egg events can appear depending on the "Fun" value written in your ini. Unfortunately, most of them were case-sensitive; since they were looking for a particular "Fun" value, they would completely ignore your "fun" value, unless you manually went in and edited it. Fixed in 1.001.
    • "abc_123_a.ogg" is an exploited example; dataminers would see it at the top of the music listing and find a message telling them to "have some respect and don't spoil the game". "abc_1111_0.png" is a similar warning in the sprite sheets, but is less easily found due to the way Game Maker works. Both were replaced with unrelated nonsense in 1.001, as Toby has since softened his stance on that kind of thing.
    • invoked. Toriel has a sprite that is similar to her defeated battle sprite and the file name has suicide in it, which implies that Toriel might've killed herself after the battle or sometime later.
    • There are seven variations of the main menu theme, but only six are used in-game. The last of them — essentially the fifth menu theme with an added glockenspiel — is found between the used "Alphys" and "Pacifist Ending" versions of the song, the fifth and seventh variants, respectively; this implies it was meant to be used for Alphys, and another friendly character (most likely Mettaton, who can't be befriended in the final game) would have used the fifth version instead. It goes unused simply because there's nowhere for it to be used.
    • invoked. Two pieces of unused text have been found. One involves Mettaton sending you a "Mortal Enemy Request" if you deny Napstablook's friend request in Hotland (in the final game, you get the same response as if you'd accepted it); the other involves a history book entry defining what is meant when a monster "Falls Down" (while the concept is only ever mentioned in passing in the final game, the definition — comatose and near death from old age — is in line with what fans suspected).
    • A special song called "Star" was meant to play if you endured Madjick's chaser orb attack long enough for the attack to turn into hearts that would restore your health. It was cut due to resulting in glitches with the normal CORE background music.

    E 
  • Early Installment Weirdness: The Kickstarter videos have some different characterizations from the actual game. Sans is seen closing his mouth to drink lemonade, while he never closes his mouth in the game, sporting a goofy grin at all times. Toriel is also shown manifesting lemonade in thin air, though the only magic she's shown to have in the game is fire magic.
  • Earn Your Bad Ending:
    • Getting a Game Over in the Boss Battle against Toriel takes some effort. You really have to be trying, since her attacks will cancel early if you have less than half health at the start of an attack. If you are one hit away from death, her attacks will actively avoid you. With 11 or 12 HP, you need to run into three fireballs before her attack ends. Although Jacksepticeye managed to do so by accident, with 8 HP and still having the Bandage equipped, he got hit by two fireballs in a row.
    • A No Mercy run requires you to do much more work than any other run, killing every random encounter in each area (all of them; you won't discover there's a finite amount unless you're a habitual grinder). Other than that, it is way easier than a neutral or pacifist run because most bosses get one-shotted by your evil and hatred, and several puzzles are solved for you, but has some tremendous Difficulty Spikes in Undyne the Undying and Sans. All this for the worst ending in the game.
    • If you want, you can do the No Mercy run again after you do it once, with the same results; the Fallen Child even questions you for it. Doing a Pacifist run after you sell your SOUL to the Fallen Child isn't any better.
  • Earn Your Happy Ending: On the other hand, if you spared every enemy and got to the final room with no kills? Not only did you not increase your LV (meaning you beat the game on the starting 20 HP), you still don't get the good ending unless you've befriended everybody. Then after that, you go through the True Lab, and found out the secrets of the hidden lab, as well as the truth of Flowey's existence, followed by another boss battle.
  • Easter Egg: There are plenty! But they're spoilerly enough to have their own page.
  • Easy Levels, Hard Bosses: To a heavy degree. Standard enemies, while not effortless, can usually be spared either instantly or after a single ACT, with only a few exceptions, and attacking them will bring them down pretty quickly, especially if you've gotten good with weapon timing and have higher LV. Bosses (at least the ones from Papyrus and beyond) are another story; they have high health, they employ Bullet Hell with their attacks, they mix up the dodging system, and Sparing them requires either a thoughtful series of actions or dragging the fight out for quite a while (over 20 or more turns for the major bosses; most enemy battles will take 2-4 turns at the most). This is especially true in the No Mercy path; you can plow through enemies with ease, but anything that can take more than a hit from you is far harder to beat than anything on other routes.
  • Easy-Mode Mockery: Inverted. Hard Mode is the joke game.
  • Effortless Achievement: The PlayStation 4 and PlayStation Vita versions are mostly just taking the piss when it comes to its trophies, aside from a few fairly standard "reach <this specific area>" trophies. A few of them are literally just joke trophies acquired by picking up your first four items, and the rest is just donating gold to the Dog Shrine, up to 350G, which isn't even remotely difficult, it's just a matter of knowing where the shrine is in the first place. You can get a platinum without even finishing the game. Considering Toby Fox outright worded the announcement of trophy support as an apology, Sony mandates trophy support on both systems, and achievements clash with one of the game's themes, it makes sense that he'd go out of his way to not only poke fun at the system, but also rob trophy hunters of any sense of achievement.
  • Egopolis: In the Neutral ending where Mettaton becomes the new ruler of the Underground, he turns it into this.
  • Either/Or Prophecy: The Delta Rune in Waterfall tells the prophecy that an Angel will come down from the mountain and, "the Underground will go empty." Either the Angel will destroy the barrier and free monsterkind from imprisonment, or the Angel of Death will slaughter everyone.
  • Empathic Environment: The save point immediately before the battle with Undyne reads "The wind is howling. You're filled with determination..." If you kill her, the wind stops, and its message changes accordingly.
  • Endgame+: While you can't reach the true pacifist ending the first time through the game (since you're required to get a neutral ending first), if you do a pacifist run first anyway, it's possible to reach said ending just by reloading the save and backtracking a bit (and doing the date scenes with Papyrus and Undyne if you hadn't already), rather than needing to start from the beginning.
  • Enemy Roll Call: The credits names off all the enemies, who designed them, and what happened to them in the end.
  • Enemy Scan: The "Check" ACT, available for almost every battle, gives a brief description of the enemy and their attack and defense. While it isn't too helpful, it does give a bit of insight on some of the characters, and reveals some odd details (namely that the hardest boss in the game has the worst stats of any enemy, because he doesn't need anything better). It doesn't work on every battle, however, most noticeably with the Amalgamates, who are too terrifying and alien to be described with a simple check; the game doesn't even provide the option for some of them. For some battles, it even lies to you. note  Oh, that enemy has 5 attack? NOPE, it has twenty. This can only really be found through the game code, though.
  • Epic Fail:
    • You can "lose" the fight against the tutorial dummy, an "enemy" that does absolutely nothing. Should you continuously miss your attacks or spare it repeatedly, it will "get tired of your aimless shenanigans" and just float away.
    • Undyne's attempt at cooking, or more specifically, teaching you how to cook, ends in her house catching fire. It's still on fire during the Playable Epilogue.
    • The Lesser Dog spends the entire game after its encounter playing poker against itself and losing. No, not Solitaire or some other one-player card game. It's playing poker, a game in which victory or defeat is completely a matter of chance (and face-reading, deception, confidence, and misinformation gambits, but that doesn't really apply since it's playing against itself). And it's losing. Somehow. It doesn't manage to "win" its one-dog poker game until the Playable Epilogue.
  • Epileptic Flashing Lights: Replaces the fakeout Defeat Equals Explosion when the human SOULs are dispelling Flowey's "Photoshop Flowey" form to finish him for real at the end of his boss battle.
  • Epiphany Therapy: Subverted with Alphys in the Pacifist run. Although she confesses to Undyne that she feels like a fraud and has lied about anime, and Undyne tells her she loves Alphys Just the Way You Are and enacts Tough Love by making her run with Papyrus, Alphys still doesn't feel great. You find a note before entering the True Lab that reveals she might not come out, which means that by following her, you could potentially interrupt her off-screen suicide because she protects you from the enemies within the True Labs. This is Truth in Television; someone suffering social anxiety and depression won't just get better and may even feel lower after a high of hearing someone cares about them.
  • Eternal Engine: The CORE, a high-tech center that serves as the power source of the Underground. When the player arrives, it’s been taken over by Mettaton, who leaves traps and mercenaries everywhere.
  • Even Evil Has Standards: The Fallen Child voices their disgust towards you upon completing a second No Mercy route, saying that you are "wracked with perverted sentimentality". They also seem rather displeased with you if you think you're above the consequences of your actions. It's possible to interpret their "tainting" of the Pacifist route as them punishing you for what you've done in No Mercy.
  • Everyone Can See It: During Mettaton's quiz, he asks who Alphys' crush is. If the player responds "Undyne", she blushes and Mettaton says "I told you it was obvious. Even the human figured it out." Even more so in her date sequence where she attempts to offer you metal polish, scale cream, and a spear repair kit. One guess for who those are meant for.
  • Everybody Lives: The True Pacifist ending naturally ends with everyone in the Underground alive. In fact, everyone being alive is important to the end of the run; Flowey has to absorb all the monsters' SOULs to gain his maximum power, so killing even the least important of monsters locks you out of it (though the direct reason is that you can't get Undyne's Letter if there have been any casualties).
  • Everything Fades: Monsters turn to dust when killed, explaining why they vanish after being fought. This is notably incorporated into monster funerals and comes up a few times in the plot of the game.
  • Evil Is Not a Toy: One of the major plot points is that you can go back and change your mind about doing almost anything. However, if you complete a Genocide route, it will permanently ruin the happy ending of any following Pacifist run, implying that the Fallen Child will kill off the other characters later.
  • Evil Is Easy:
    • EXP can only be gained by fighting enemies; sparing them only gives you gold. Of course, this means that if you don't fight anyone, your life bar will never get bigger...
    • Taking the full No Mercy route eventually subverts this. The amount of encounters in this route that don't go down in one or two turns can be counted on one hand. But those encounters are the most challenging in any route of the game.
  • Evil Only Has to Win Once:
    • In full force on a No Mercy run. The few characters who can stand up to you will most likely kill you over and over again... but you have determination on your side to bring you back from the dead and they don't, so unless you reset, they're ultimately fighting a hopeless battle. Likewise, completing a No Mercy run just once is all the Fallen needs to destroy the world and steal your SOUL, which negatively affects all subsequent True Pacifist playthroughs.
    • Inverted in a No Mercy run, as you only have to show Mercy once to lock yourself out of it. note  When you discover the consequences of your murderous playthrough, you might reflect on how you made a willing effort to bring them about. You could have made that world less miserable any time you wanted... and it would have been so easy.
  • Evolving Music: As you befriend more and more characters, the Main Screen's music becomes more lively as more and more instruments are added in. Completing the True Pacifist run and seeing the epilogue turns it back to zero, as everyone had gone up to the surface at this point within the game.
  • Evolving Title Screen: The Main Screen will be filled with characters that you've elected to befriend throughout the game. Completing the True Pacifist run and seeing the epilogue turns it back to zero, as everyone had gone up to the surface at this point within the game.
  • Exactly What It Says on the Tin:
    • "Song That Might Play When You Fight Sans." It doesn't play when you fight Sans; the operative word was "Might".
    • "Bird That Carries You Over A Disproportionately Small Gap" describes the exact context this song plays within.
    • The "Conveniently Shaped Lamp". It's a lamp, conveniently shaped to "perfectly" hide the protagonist.
    • Several of Asgore's names for places are quite self-explanatory:
      • "Home" is the monsters' (original) home. This name mostly applies for the backstory. By the time Frisk ends up in the Undergound, it's taken on another name: the Ruins.
      • "Waterfall" is a cave full of waterfalls.
      • "Hotland" is a cave full of lava and fire.
      • "New Home" is the new and current home for himself and the monster capital.
  • Exact Words:
    • The Tag Line for the game: "The Friendly RPG Where Nobody Has To Die." Sure, nobody has to die...but anyone can.
    • One right when you start the game: "Name the fallen human." They don't tell you which fallen human you're naming...
    • On the screen that tells you the controls for the game upon starting a new SAVE file, the last line states that you lose when your HP reaches zero. The last attack the True Final Boss of the True Pacifist route uses gradually chips down your HP, but it does so to the point where you're left with one-billionth of a Hit Point when he gives up.
    • Flowey tells you towards the beginning of the game: "I am the prince of this world's future". He's correct, but not in the sense of a pseudo-philosophical hyperbolic Badass Boast. He is literally the future (the eventual fate) of the prince of this world (Asriel Dreemurr).
    • "Take 'one'." from the monster candy bowl. Take more than one, and the game will call you out for it.
    • "Three out of four gray rocks recommend you push them." The fourth rock isn't as much of a pushover — as in, you literally can't push it, because it won't let you. You have to ask it to move.
    • "here goes nothing." Spoken by Sans before doing exactly that: nothing.
    • "trust me. there's no way they can get past this one." Said by Sans when you're faced with the Monster Kidz Word Search, which he lazily set up for a puzzle. He's actually right; you can't get past the puzzle by solving it because of a single-letter difference in the word search. He never said that you couldn't just get past it by ''walking'' past it.
    • If you killed Lesser Dog, you can reach the fire exit at Grillby's, but can't actually use it because you're "not made of fire".
    • At one point, Sans warns you that his brother has a Special Attack. The first attack Papyrus puts emphasis on in his fight is his Blue attack, not necessarily his special attack, which will probably further trick people into thinking Papyrus is all bluster before he turns their SOUL blue.
    • "SANS! PLEASE PICK UP YOUR SOCK!" "ok." "DON'T PUT IT BACK DOWN! MOVE IT!" "ok." "YOU MOVED IT TWO INCHES! MOVE IT TO YOUR ROOM!" "ok." (It just goes on and on from there.)
    • "Three gold for the ferry." The rock will then pay you. "Thanks for stepping on my face."
    • During Undyne's fight, "As long as you're GREEN you CAN'T ESCAPE!" But when you turn RED again...
    • "Song That Might Play When You Fight Sans." Heavy emphasis on "Might": It doesn't actually play when you fight Sans. "MEGALOVANIA" plays instead.
    • Sans is described as "the weakest enemy," with only 1 ATK and DEF. Looking in the game's code reveals that not only is your Enemy Scan not deceiving you here, he also only has 1 HP. His stats do accurately describe his capabilities: his attacks only do 1 damage, and a single hit will kill him (like anything else on a No Mercy run). However, just try surviving long enough to actually hit him. And when you do get the chance, he'll casually dodge your attack; the battle turns out to be a sheer endurance match, as you'll quickly find that your only option is to keep trying (and failing) to hit him until he's too exhausted to dodge anymore. Also, while his attacks do indeed do 1 damage, there is No Mercy Invincibility in this battle, and every hit poisons you. Even a slight scrape can shave off a lot of health. (So, again, good luck surviving long enough to wear him down.)
    • When Sans offers to spare you, he says that "[his] job will be a lot easier." And if you accept, he says he "won't let it go to waste." Turns out, he was referring to his job of killing you, and he won't waste an opportunity to do so.
    • One of the last history messages in Waterfall says that, to break the barrier, the power of seven human souls is needed. Combine this with an earlier message that says it would take nearly every monster to equal one human soul and the fact that Asgore already has six souls, and you get Flowey's backup plan when he realizes that he can't steal your soul.
    • When Alphys reveals that it takes at least a human SOUL and a monster SOUL to cross the barrier, she laments that in order for you to leave, you'd have to kill Asgore. Toriel is also aware of the minimum amount of power, which she points out to Asgore that he could've freed everyone easily years ago if he just used one SOUL to leave and find the rest outside, since Alphys's fact also works in reverse.
    • Against the Bonus Boss in the Nintendo Switch version, Mad Mew Mew, you have the option of employing this yourself, which gets some Lampshade Hanging if you do. Specifically, Mad Mew Mew asks you to show her what LOVE is. She means literal love, but if you employ the other meaning of LOVE used throughout the game — which is to say, violence — she remarks, before falling apart, "I suppose I should have expected that."
    • Papyrus' "invisible electricity maze" works that if you touch any "walls", you'll get zapped by an orb you're supposed to hold. The moment you walk into the maze, you trigger the orb and it zaps Papyrus instead since as Sans points out, he was still holding it and was supposed to give you the orb at the start.
    • In Snowdin, Doggo informs you that Greater Dog writes "the most beautiful letters" and shows you a message written by Greater Dog. It's just "bark" in a very fancy script, i.e. Greater Dog writes with beautiful letters.
  • Experience Penalty: If you reduce Napstablook's HP to 0, they reveal they were lowering their HP on purpose to make you feel better, since they can't die on account of already being a ghost. At the end of the fight, you lose 1 experience point. But the extra-observant may notice that you don't lose any EXP...
  • Experience Points: You earn EXP when you kill an enemy. Get enough EXP, and your LV/LOVE increases. Near the end, Sans explains that EXP and LV/LOVE actually stand for "execution points" and "level of violence".
  • Explain, Explain... Oh, Crap!: Near the end of the Genocide path, Flowey appears and talks about how your determination — the power to cheat death — has surpassed even his, then talks about how similar the two of you are in how you're both willing to kill the other if they got in your way. His expression quickly changes from his usual smug self to that of absolute horror as he puts two and two together...
  • Expository Pronoun: The Japanese translation uses pronouns to emphasize the characters' personalities:
    • Flowey uses the non-threatening "boku" and "kimi" because he hides his murderous intentions behind a happy mask. However, he will address you with the harsher "omae" when you piss him off enough.
    • The Hot-Blooded Undyne uses the haughty "kisama" for the player character because of her disdain for humans. However, she refers to herself with the neutral "watashi" instead of the harsh "ore" to reflect her dignity as a member of the royal guard.
    • Sans usually addresses the player character as the familiar "anta" when he's being goofy, but switches to "omae" when he needs to be intimidating. He refers to himself as "oira", which is usually associated with bumpkin types, which Sans isn't. However, it fits his character on a meta level: It also shows how Sans tries to project a loser-ish, slacker image. During the "Lost Souls" fight and segments before he fights you on the worst route, he switches to "ore".
    • Papyrus refers to himself as "ore-sama" (i.e. adding an honorific to the pronoun) because of his massive ego.
  • Eyepiece Prank: If the player character looks through Sans's telescope in Waterfall, they only see a solid red color. When you exit the telescope, there is a purple ring around their eye.
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    F 
  • Fake-Out Opening: It's shown that the opening cutscene isn't the protagonist, Frisk, falling down and entering the underground — rather, it's the Fallen Child who befriended Asriel. Subverted a bit in that it's not the game faking you out, necessarily, but rather your own assumptions about how RPGs work and introduce information, which are then repeatedly tested throughout the game.
  • Fake Trap:
    • The corridor of spikes can't harm you because the spikes that aren't part of the correct path only act as barriers. You wouldn't realize this at first because Toriel leads you through.
    • None of Papyrus' traps can actually hurt you. The only trap which seemingly can hurt you is his "invisible electricity maze" if you touch any "walls" while holding the orb. The worst it does when this happens is that you'll get shocked and have to walk away from that area. You'd have had to do this deliberately if Papyrus didn't leave his footprints in the maze to give you said orb in the first place.
  • Fantastic Aesop:
    • The Genocide route is extremely unrelenting in hammering the point home you're likely only taking that route to see what happens, rather than any actual desire to hurt or punish the characters, and the few characters who are aware that you're able to effectively time travel using the ability to SAVE argue the fact you can undo everything doesn't make you any better of a person. However, the only reason their point sticks is because the Genocide route ends with an all but literal Diabolus ex Machina in the form of the Fallen Child, who destroys the world whether you want to or not and then only allows you to recreate it if you sell your SOUL to them. Doing so prevents you from ever achieving the Golden Ending, as the child takes you over in the final scene of the Pacifist route and is implied to kill everyone again anyway. So in some ways, it's an Arbitrary Rules Aesop about Demonic Possession and deals with the devil, but since its primary purpose is to drive the point the other characters are making, it makes you wonder how effective their point would have been without the Child's cross-timeline possession of you.
    • Less Anviliciously, the fact that the Golden Ending can only be achieved by refraining from committing the Crime of Self-Defense no matter how violent your enemy is only makes sense in-universe because the barrier needs seven human souls worth of power to be destroyed; the six human souls in reserve plus nearly every monster in the Underground conveniently make up the correct amount, and you can't do it with one iota less. It's a pretty convenient coincidence that this is the number of monsters currently alive, and the fact that almost every living monster soul adds up to one human soul is only mentioned on one Pamphlet Shelf.
  • Fantastic Racism: Zig-Zagged. Humans' fear of monsters led to the war, the monsters being trapped underground, and, you know, the whole plot, but Word of God says that after returning to the surface in the Golden Ending, the monsters will be fine, implying that most of humanity has gotten over it. Some monsters — Undyne in particular — aren't too fond of humans, either, but every monster in the Underground is willing to let you go with no further hassle, and even befriend you, if you prove you have no intention of hurting them. On the other hand, there are some monsters that don't realize you are human. On the other other hand, even the ones that do realize it can be befriended if you're nice, Undyne included. And even Undyne has an appreciation for human culture. To put it simply, monster-human relations are complicated.
  • Fantasy Kitchen Sink: The setting of Undertale is a place where you can meet a pair of comical skeleton brothers, a genocidal monster flower, a middle-aged pie-baking goat lady, an anime geek dinosaur scientist and a Wrong Genre Savvy Hot-Blooded fish knight.
  • The Farmer and the Viper:
    • In the Neutral ending, you have the option to spare Flowey. He not only fails to understand this, but blatantly comments that refusing to put an end to him just means he will continue to torment you and everyone you love. Afterwards, it seems to have somewhat affected him, causing him to reveal the way to get a truly happy ending... only for it to turn out to be a ploy to bring every SOUL into his grasp. And then subverted — after said SOULs remind him of what it's like to feel love (not to mention making him capable of actually feeling it again), he immediately throws out his previous plans of destroying the world in favor of a Stable Time Loop that will allow him to play with you forever, then throws that plan out once he realizes he's being selfish and self-defeating. Even after he releases the SOULs and returns to his original state, he shows remorse for what happened even given his returned lack of love.
    • You in the No Mercy path. Toriel will realize this if you attack her after sparing her.
    • It's also an important part of the backstory. After being taken in by Toriel and Asgore, the "Fallen Child" attempted to manipulate their adoptive brother Asriel into murdering the fallen child's entire village. The end result was that the two of them died, with Asriel being reborn as Flowey. The Fallen Child shows no remorse for this, and on a No Mercy run, they'll gladly see both of their adoptive parents killed. Although this could be subverted, depending on how you interpret the Fallen Child's motivations and their relationship with their adopted family.
    • Within the No Mercy path, it's implied that Flowey's been helping you on your journey by, among other things, solving puzzles for you, just so that you could focus on your goals of destroying everything in your path. You eventually repay his help by killing him. Though it could also be interpreted as the child punishing him for his betrayal, as he will be found trying to warn Asgore about your arrival, then when that fails and Asgore gets one-shotted, Flowey finishes Asgore off (which, in this context, comes across as a Mercy Kill), then shatters his SOUL, leaving the child stuck underground.
  • Faux Adventure Story: The game looks like a very typical 8-bit RPG with the classical "boy gets lost in the world of monsters" plot... But the thing is, to get the best ending, you have to not kill monsters, so monsters and humans eventually make peace. Otherwise, if you play it like a classical RPG and kill all the monsters for points, you get a bad ending where you become a monster.
  • The Ferryman: The river person, who, after you reach Waterfall, will allow you to quickly travel between Snowdin, Waterfall, and Hotland, the game's three main areas.
  • Fighting Your Friend: Part of the fight with Asriel on the True Pacifist run involves liberating your friends from his control.
  • Final Boss: Photoshop Flowey on Neutral and Sans on No Mercy. Once Photoshop Flowey has been defeated once, Asgore takes his place as final boss of the Neutral route. Asriel, while the True Final Boss of the game as a whole, is technically this for True Pacifist, as well.
  • Final-Exam Boss:
    • Once you make it to the capital, the encounters include elements from previous bosses.
    • A more straightforward example comes from the Pacifist route's Final Boss, in which saving the Lost Souls who happen to be your friends requires you to use almost every gimmick the main bosses threw at you, such as jumping over obstacles with the blue SOUL. In addition to this, you also have to avoid familiar attack patterns which depend on who you're trying to save.
  • Fisher King: New Home is grey to represent Asgore's mood after the death of his children.
  • Fission Mailed: The game pulls this a few times, but especially towards the end of the Neutral Route, where you are required to get a bad — though not the worst — ending note . See also Death Is a Slap on the Wrist — the worst you will get for dying is being put back at the last SAVE checkpoint you visited and you may lose some money.
  • Final First Hug:
    • If you spare Toriel, she hugs the protagonist before they part ways.
    • Also with Asriel if you choose to comfort him in the Pacifist ending.
  • Flat "What": Undyne gives one during her rematch against you when you pretend to attack her. Said attack leads to the screen flashing white... before revealing that you only dealt 1 damage.
  • Flawless Victory: If you dodge all of the names in the special backer credits, you'll gain access to the Developer's Room.
  • Flower Motifs: Golden Flowers, or Buttercups.
  • Foolish Sibling, Responsible Sibling: Played with when it comes to Sans and Papyrus. On first glance, Sans seems like the responsible one, wanting to keep you around to make Papyrus happy, while Papyrus seems quite silly and clumsy, messes up the electric maze puzzle, refuses to take off his "battle body" (read: costume made for a party some time ago), and seems like he actually doesn't have a clue what he's doing. However, in the boss battle, the true nature of his hyped-up "blue attack" is revealed — he turns your SOUL blue, introducing a completely new game mechanic, which he promptly demonstrates by sending out one small bone to hit you at the bottom of the box (while you're probably still completely confused about what just happened and have yet to realize that your movement mechanics have switched from "top-down free-roam" to "2-D platformer"). The boss battle also makes it clear that Papyrus is actually extremely in control of his strength — while even Toriel can accidentally kill you, Papyrus will immediately stop to capture you if you hit 1 HP. The only other monster shown to be capable of toning down their own strength and halting their attack to avoid reducing your HP directly to 0 is Asgore himself. And hell, when Papyrus can't use his special attack, he uses "just a really cool regular attack" — which manages to be one of the hardest attacks in the entire game — near the end, it's technically impossible to dodge if you don't hold the jump key so that your SOUL rises upwards and stretches the "bullet board" so you can dodge it. Undyne points out that he's actually a lot stronger than he leads on, and he consciously makes the decision to become your friend because he trusts you and believes in you, not because he's naive. Sans, on the other hand, spends most of his time at Grillby's, has an extremely messy room (not even a proper bed), the only thing on his side of the fridge is an empty bag of potato chips, and his performance at work is only just good enough not to get fired. And while he's very popular in town and everyone knows him, aside from Papyrus, Toriel, and possibly Alphys as is revealed in the Pacifist ending, he has no real friends. In the end boss battle, he reveals that he most likely has really bad depression. If you kill Papyrus on a neutral run, he will call you out on it — no matter if you kill or spare countless other monsters along the way, you will only be a "dirty brother killer". He won't even bother judging you. He completely depends on Papyrus because after all the resets, he's all he's got.
  • Foregone Victory:
    • Until you face him as the Hotlands boss, it's actually impossible to face any major consequences from Mettaton's challenges. Either Alphys will save you regardless of what happens, or it's set up so you can't actually lose. Mettaton later reveals that Alphys did this on purpose so she could be involved in your adventure.
    • On the No Mercy path, it is impossible to lose to Mettaton NEO. Not only does he die in one hit like most other bosses along this route, but he does not take any action at all and therefore cannot hurt you.
    • The true ending's final boss is impossible to lose to. You are so filled with DETERMINATION that when your health depletes, your SOUL will split in half... then come right back together again with the words "But it refused".
  • Foreshadowing:
    • The demo's manual page about enemy turns has been ruined by "artless hooligans", according to Flowey. Turns out it was probably him, so he could trick you when you first meet him in-game. He can still change the manual after you finish the demo.
    • If you put in Flowey's name when you're naming the fallen child, he'll say "I already CHOSE that name," hinting that he was able to choose his own name rather than be born with it.
    • The very intro of the game, the first thing one sees upon launching the game, has subtleties which show that the child that falls in the intro is not the player character: the stripe pattern on Chara's shirt is visibly different from Frisk's and Chara falls onto bare ground whereas Frisk falls onto golden flowers. The intro also features a silhouette of Asgore fighting. Anyone familiar with goat biology who happens to remember this image might figure out that Toriel is the queen far before other players.
    • Near the end of the Ruins, the player can find and equip a toy knife. Playing violently up to that point will also cause the narration to ask where the knives are in Toriel's kitchen. And with enough effort, you can indeed find one.
    • If you talk to the Snowdin shopkeeper about the Ruins, she says, "unless you're a ghost or can burrow underground, it's impossible to get in". A miniboss fought there is a ghost. Sure enough, said ghost is the only monster besides Toriel that you can meet outside of the Ruins, or, more specifically, Waterfall. In addition, Flowey is also shown being able to burrow underground...
    • The entire scene with the training dummy at the start carefully picks its descriptions to personify an inanimate object. Because it's actually the cousin of the Mad Dummy you fight later on.
    • The last weapon you can get in the game is the Worn Dagger. Its description reads "good for cutting weeds"... you know, like Flowey.
    • Napstablook doesn't have a "damaged" sprite or sound effect. Your attacks aren't actually doing anything, and they're just reducing their HP out of politeness. After "depleting" their health, they leave, and because the battle accomplished nothing, the game tells you, word-for-word, that "You lost one experience point." Observant players will notice that the game specifically says "experience point" here, while normal battles call it EXP; extremely observant players will check their status menu and notice that they didn't actually lose any EXP. This is because "Experience Points" don't exist; EXP, or "Execution Points", is just a term coined by monsters for how someone's ability to kill develops. You don't learn this until Sans's judgement at the other end of the game.
    • In Toriel's home, there is a calendar with a date in the year 201X circled. This is the date that Chara fell.
      • Said calendar is described as old, hinting at the later reveal that 201X is not the year the game takes place.
    • Both Mettaton and Alphys drop hints that Mettaton being a malicious killer is actually a lie to get you to like her. This includes Mettaton drawing out and repeating lines at Alphys' cue which she frequently misses, Mettaton's robotic cough and stopping the 'firewall' before it hits you, him lengthening the bomb defusal time, one of his news ticker headlines lampshading the malfunctions and puzzle reactivation in Hotlandnote , frequently praising Alphys, and picking up her lines when she forgets them.
    • At certain points in the game, if you quickly return to the start of the room you're in, you'll catch sight of Flowey quickly receding into the ground. He's been following you this entire time, foreshadowing his sudden appearance at the Barrier. You led him safely right to Asgore.
    • Loads towards the identity of the king and queen. Toriel wears the Delta Rune on her dress, foreshadowing her place as royalty. A Froggit in the Ruins comments that everyone in the Ruins is "too intimidated" to talk to Toriel, hinting early on that she's more than just an ordinary citizen of the Underground. Asgore uses attacks identical to Toriel's, just more powerful. The royal castle is a one-to-one copy of Toriel's home. An incredibly subtle hint is the description of Toriel's bed: "Definitely bigger than a twin-sized bed.", which would make this around a queen size. Returning in the walkaround section after the Pacifist ending (after the relevant revelation) will change the text to "Earlier you identified this as bigger than a twin bed. You realize now it's one more size above a double." In other words, a queen.
    • Flowey's theme is titled "Your Best Friend," which fits his Faux Affably Evil nature. This foreshadows his true identity as Asriel Dreemurr, prince of monsterkind, and best friend of the Fallen Child, with whom the player likely happens to share a name. On a Genocide run, the Fallen Child possesses Frisk because of the player's influence, making the title "your best friend" even more literal.
    • One of the Froggits in the Ruins warns you that someday you might have to spare someone even if their name isn't yellow (an indicator that you can spare a monster). This is the key to getting through the fight with Toriel without killing her.
    • During the No Mercy run, when you first encounter Sans and don't laugh at his joke, he mutters to himself, "gee, lady, you really know how to pick em, huh..." At first it seems like he's talking to you, but really, he's talking about Toriel, who asked him to spare any humans he encountered. Due to the nature of the run, however, you wouldn't discover this unless you had previously done a Pacifist run (though it's heavily implied in an optional conversation with Sans at the MTT resort, in any run where Papyrus was spared).
    • When you're on the date with Papyrus, you find a joke book his and Sans's home. Inside the book is a quantum physics book, and inside that is another joke book, and inside that is another quantum physics book, at which point you decide to stop. Though it doesn't make sense out of context, this proves that Sans uses the quantum physics book to grasp the concepts that allow him to use his time machine, and the multiple layers of books within books shows that he's able to send objects back in time, just like he did with the photo of the playable character and the NPCs after the post-pacifist true reset, as he was able to retrieve multiple copies of the same books. Then again, Word of God states that he was never able to fix the broken time machine (and never will be able to), so this symbolism may not quite work.
    • In Papyrus's culinary art museum, a.k.a. his refrigerator, you'll find lots and lots of containers labeled "spaghetti" and a single, empty bag of chips. The only other place you can find chips is in the True Lab. This foreshadows Sans's position as a scientist and, by extension, his connection to Dr. W.D. Gaster.
    • When Sans warns you that you will have a bad time and disappears during the No Mercy run, it is a strong implication that Sans is self-aware and can access the same meta-concepts as the player, as no other character has been able to simply teleport out of nowhere and reappear at will. He can also do this during the Pacifist run, when you are trying to solve a puzzle at the bottom of a cliff and Sans is at the bottom, waiting at both ends of the screen although we never see him move. He lampshades this by asking if you are following him, and stares straight at the player instead of at Frisk while he's idle.
    • The second time you meet him in the Ruins, Flowey refers to himself as "the prince of this world's future". His true identity, Asriel Dreemurr, is the prince of monsterkind.
    • When Sans is talking about his brother's "special attack", the term itself is highlighted in blue. Actual blue, not the cyan color which Sans uses to talk about "stop signs", so by that logic this "special attack" is something you've never seen before. Cut to the actual boss fight with Papyrus, he reveals that he can subject the SOUL to gravity... by turning it blue.
    • An odd retroactive example: during the battle with Asriel, when you try to SAVE Papyrus and Sans, if you SAVE Papyrus first, he'll fall silent, but the attacks will keep coming, hinting that Sans is more powerful than he first appears.
    • At one point in the game, you can get an optional conversation with Sans that reveals Papyrus receives flattery, advice, encouragement, and predictions from Flowey. This foreshadows the method Flowey used to trick everyone into coming together during the Pacifist route.
    • When you first meet them, Monster Kid remarks that you must be a child since you're wearing a striped shirt. Turns out this character design convention is true for the First Child and Asriel.
    • If you end Muffet's fight without killing her, she'll come to the conclusion that her beef with you is a big misunderstanding, and that she thought you hated spiders. She decides that the person who wanted her to steal your SOUL must have been referring to a different human in a striped shirt. The First Child hates a lot of things. Or that either Flowey or Mettaton, both of whom change shape, is a dirty liar.
    • The Monster Kidz Word Search, a simple gag in Snowdin, has a few subtle hints towards future events. Not only does it have the four seasons in it (see Seasonal Baggage below), but it also contains "skeletons", "mermaid", and "robot"; while you already know about the skeletons at this point (they're the ones who set up the word search, after all), this is far before you encounter Undyne (who isn't a mermaid, but is still a fish-based monster) and Mettaton (the only robot in the game).
    • While poking around Alphys' lab, you can find a half-filled bag of dog food, and a stack of unopened letters from monsters like Froggit, Snowy, and Doggo. Nothing suspicious since there's dogs everywhere in this game and Alphys is an anti-social shut-in, right? If you explore the True Lab, these innocuous items become a lot less innocent — Alphys uses dog food to feed the various Amalgamates sealed down there, and is so guilt-ridden about accidentally creating them that she can't bring herself to talk with the horrors' families.
    • In the MTT Resort, the fountain's plaque says it was built in 201X and the Mettaton statue was added last week (that is, quite some time after 201X). This foreshadows the fact that 201X was decades (or possibly centuries) ago, rather than being the year the game takes place in.
    • If you speak with the various people inside the MTT Resort's restaurant, you learn that the Core can be configured in multiple ways, and that a new configuration has just been finished. Alphys may have intended the Core to be configured specifically for the player, but the fact that the configuration doesn't match her map suggests that someone has messed with it. This turns out to be Mettaton's doing.
    • Upon reaching the palace, if you keep going past the throne room entrance and instead head down some stairs, you'll find a few coffins, one of which has whichever name you've input on it. The initial thought is that at least Asgore was kind enough to prepare your burial for you, but this line of thinking disappears when it's obvious that he has no idea who you are. This foreshadows the Fallen Child's actions and the twist involving them.
    • Speech patterns can reveal a lot about an individual. For example, there are only two creatures in the underground who greet people with the word "Howdy": Asgore Dreemurr and Flowey. In hindsight, this becomes quite an obvious hint that Flowey is actually Asriel Dreemurr, Asgore's son. The one time Papyrus says "Howdy" is when he's secretly being led by Flowey.
      • Additionally, two characters will greet you with the word "Greetings", Toriel and Chara.
    • After realizing that you can't hurt each other, the Mad Dummy threatens to keep you trapped there by never ending the fight. Guess what Sans tries at the end of your fight with him in a Genocide Run?
    • There are multiple subtle things in the True Labs that foreshadow the plot twist that Frisk is their own person, not just a player insert. At three different points, if you tell them to do something, they'll behave erratically or without your input. When you approach the shower, they move slower, showing they're afraid. When you try to laugh at the Snowdrake Amalgamate, they instead burst into tears (which is lampshaded, "what, you didn't do that?"). Lastly, after finishing each of the Dreemurr family home videos, they'll turn away from the TV like they know this isn't something they should be looking at.
    • The theme that plays when you're sparing Asgore is Asriel's theme. Guess who's secretly in the room right now.
    • After beating the game for the first time and sparing Flowey, he will start giving you hints on how to get the Golden Ending. Pester him enough, and he will eventually tell you to stay away from "Smiley Trashbag" (aka Sans) for causing him 'more than his fair share of resets' before. Then you go for the No Mercy route and you find out that Flowey is being completely sincere...
    • In Toriel's home, there are three different-sized chairs at the dining table. This first foreshadows that a family of three used to live here: Toriel, Asgore, and Asriel.
    • Toriel's diary is a record of bad puns, with the one we see being skeleton-themed. In the True Pacifist ending, it's revealed that Toriel and Sans held a pun-based friendship from each side of the door to the outside, so the jokes written down are her remembering the puns she exchanged with him.
    • Toriel's bedroom contains a vase of prominent yellow flowers, the first instance they are hinted to be connected to her family and son.
    • The top half of the Delta Rune and the prophecy associated with it is theorizes that either a pure savior or a dark angel holds the solution to monster freedom. In the True Pacifist ending, both are true. The player is the savior who fights Asriel, the villain seeking monster freedom, who takes on an angelic form in his battle.
    • In the news-report section of Hotland, Mettaton's blurbs give hints that all of the objects to choose to report on are bombs.
    • If you continuously call Toriel after she gives you the cell phone and tells you to stay put (but not flirt with her), she'll suggest something for you to do, such as going to a pile of leaves and pretend to rule over it with an iron fist. This hints towards her frustration against King Asgore.
    • When Gerson describes the Delta Rune, he pauses for a moment before remembering that it's the emblem of the Kingdom of Monsters. Almost as if there was another kingdom it'd be associated with...
  • Forgiveness:
    • In a True Pacifist Run, you forgive all of the Boss monsters that attack you, including the hotblooded ones like Undyne. Undyne lampshades this at the end of the game, that all the Boss monsters except Papyrus have tried to kill Frisk.
    • Zig-Zagged with Toriel and Asgore. She's still angry with him for declaring war on humans in the Underground and killing six children that she had tried to protect, since he only needed one soul to cross the barrier and obtain six more. He doesn't blame her one bit for feeling this way. On the other hand, she does save him from you in the True Pacifist ending and they work together at a school in the Golden Ending.
  • "For your people, by your people!": Parodied with the Spider Bake Sale, which offers baked goods made "by spiders, for spiders, of spiders!"
  • Four-Philosophy Ensemble: The Main Monsters you can befriend can count. This is evident if you research their behaviour in all three paths to the different endings.
    • The Optimist: Papyrus. His naive yet kind nature makes him the most friendly of them. Hot-headed and rather self-centered, but in a good way, Papyrus is also loyal to you once you befriend him. He is one of the few that even in a No Mercy Run will say upon his death that he believes that you can be a good person, showing just how deep his optimism goes.
    • The Realist: Alphys. How she acts depends on which path you're taking. She'll either consider you friendly and a kind person or a nightmare and that she avoids having to meet you in person and helps all the monsters to escape from your reach.
    • The Cynic: Undyne. The foil to Papyrus, once she meets you in person, she blames your unwillingness to die and give your SOUL as the reason why everyone is still stuck in the Underground. She mellows out significantly if you befriend her.
    • The Apathetic: Sans. His knowledge of the ability to SAVE left him broken and finds reaching to the Surface no longer satisfying. He will break this way of thinking to battle and try to stop you in the No Mercy run, however, and is implied to be in a better state of mind at the end of a Pacifist Run as well.
  • Four-Temperament Ensemble: The four monsters you befriend in the Pacifist route. Papyrus (sanguine), Undyne (choleric), Alphys (melancholic), and Sans (phlegmatic).
  • Fourth Wall Psych: There are a few moments where the game comes dangerously close to being a RPG-Mechanics Verse but doesn't quite cross that line. Many of the game's aspects that most players find part-and-parcel for RPGs (i.e. saving and loading, experience points and levels, the Silent Protagonist) are bluntly pointed out to the player, but are also given in-universe explanations (Determination, Level of Violence, Symbiotic/Demonic Possession), and the game's meta-story is an exploration of these concepts. Notably, the Pacifist ending involves inducing this, finding out about an in-universe person for Flowey to be talking to when he addresses the person controlling Frisk and turning his thoughts towards them so he can properly sort through his own emotional baggage.
  • The Fourth Wall Will Not Protect You:
    • Upon defeating Asgore, Flowey steps in to absorb the six human souls, which causes the game to abruptly quit. Upon starting the game back up, things only go downhill from there.
    • Also part of Flowey's M.O. since he also has the power of DETERMINATION. Flowey routinely jabs at you for cruel actions and laughs if you try to use the save system to wipe them clean. His biggest threats to you are also to destroy your save file and reset the game to the beginning.
    • The game dives into this full force at the end of the No Mercy run. The boss tries to convince you to stop playing, and things get even worse in the ending itself. And it gets even worse than that if you try to restart the game after that.
    • This even managed to sneak itself into the merchandise, if you buy Fangamer's "determination" bundle, Flowey will appear and offer you a one dollar discount. It's treated like a Deal with the Devil: if you accept it, the postcard that comes in the box, which is normally a detailed photo from one of the Pacifist endings, is replaced with the Soulless Pacifist ending's version in horrifying detail. Hope that one dollar you saved was worth it.
    • And if you thought that the this abuse of the fourth wall is resigned only to what the author controls, it somehow manages to break the fifth wall(?) by addressing people watching the game being played. During Flowey's monologue in New Home, he'll express distaste at people doing the genocide run "because they need to know" for trying to justify their actions with shallow excuse, but say that even worse are the people who watch a genocide run "because they need to know," i.e. let's play and stream watchers. The game is so meta, that it has the foresight to address the audience behind the audience and judge them for it.
    • Inverted in the genocide route, as some characters will express fear of the player. For example, after Flowey's monologue in New Home, he realizes that he's the player's next target. Sans also knows about the player, and it is revealed that he has lived most of his life simply accepting everything the player does, knowing that they will eventually reset anyway.
    • Sans' fight in the No Mercy run isn't designed to stop the player character. It's designed to stop the player. He knows that the player character has limitless resets and lives to spend on this and that they'll win eventually if you keep trying, so his goal is to make you so frustrated that you quit and try something easier, like the pacifist or neutral runs.
  • Fragile Speedster: If you keep the Bandage on, you'll have no additional defense whatsoever, but you'll be guaranteed to be able to run from any encounter that gives you the option.
  • Freeze-Frame Bonus:
    • If you die to Toriel, she'll show a unique sprite for a split-second depicting her shock at having killed you before you see the game over screen.
    • If you backtrack in some areas, you'll see a quick flash of... "something" disappearing into the ground. It's Flowey, stalking you the whole game and observing your actions before picking the best time to strike.
    • It's possible to talk to the Snowdin innkeeper while the screen's still fading to black after deciding to stay at her inn. Her response, in which she says, "What? No, you can't get a second key!", is this, as you wouldn't be able to read it otherwise due to the speed of the transition.
    • In the game's release trailer, Undyne is shown hunting the player, captioned with "And strong enemies." "..?" Is tacked on just before transitioning to the next scene, as she can be befriended over the course of the game.
    • If you pause the release trailer right at the beginning of the "probably not actual gameplay footage" segment, the tree in the background has one of Flowey's toothy grins on it for no particular reason.
  • Friendly Neighborhood Vampire: Apparently, humans and monsters live in harmony, and still do at the end. Think about all the monsters you meet, and think of how people would react.
  • The Friends Who Never Hang:
    • Papyrus's interactions with Alphys are limited: Beyond mentioning her as the one who designed the colour tile puzzle in Snowdin and showing up during Alphys's date to cheer her up, they don't interact that much.
    • Sans and Undyne interact even less with each other, the only time where they are seen together is when Undyne is chasing you through Hotland, seeing Sans sleep and getting annoyed for not helping her and in some of the Neutral Endings where both of them survive.
    • Also Sans and Alphys never interact much, though late in the game it is revealed that they do know each other, since she is familiar with his humor, but whatever this connection is, the game doesn't elaborate on it.
  • Frying Pan of Doom: The Burnt Pan weapon, which is even more useful on a Pacifist Run since wielding it causes consumable healing items to restore 4 more HP than normal.
  • Fun with Acronyms:
    • EXP and LOVE, of course! According to the Judge, Sans, they actually stand for EXecution Points and Level Of ViolencE.
    • In the Ruins, one Froggit that knows about timed hits comments on F4's function, wondering if it stands for "four frogs", even though there's only two other Froggits in the room with it. The fourth frog actually exists, but it's tiny and you have to interact with a specific portion the back wall to find it.
  • Funny Animal: A large chunk of the monsters are anthropomorphic animals (goats, cats, bunnies, fish, dogs, etc) with human personalities and clothing.
  • Funny Octopus: Onionsan is a Giant Squid-like creature who initially seems threatening... until they reveal their onion-shaped head and comical Animesque facial expressions.
  • Furry Confusion:
    • A rabbit lady in Snowdin has a small, non-anthropomorphic rabbit on a leash as her pet. A rabbit guy nearby finds this a little disturbing. The Playable Epilogue reveals that this leash-rabbit is actually her brother.
    • Catty also talks about how she wants to buy a cat at some point, despite being a big humanoid cat herself.
  • Furry Female Mane: Gender-Inverted with Asgore and Toriel. While Toriel doesn't have feminine hair, and her only Tertiary Sexual Characteristics are eyelashes, Asgore spots a long viking-like blonde hair and a beard to highlight his manliness.

    G 
  • G-Rated Drug: Doggo likes to smoke dog treats. Likely the reason why the ESRB rating mentions "Tobacco References".
  • Gag Dub: Undertale With Voices: Genocider and Undertale With Voices: Pacifist, both brought to you by The With Voices Project.
  • Game-Breaking Bug:
    • Well, diminishing the experience of one small part of the game, at least — Muffet's "pet" attack causes severe slowdown on some systems, and nobody is quite sure why.
    • Entering "Gaster" as your name in the PS4 and Vita versions will crash the game, rather than restarting it like in the PC versions. Given the nature of W.D. Gaster, it's quite likely this is intentional.
  • Game Gourmet: This being a Spiritual Adaptation to Earthbound, there is a plethora of different foods you can eat, all of which have Punny Names and descriptions. Without spoiling anything, see for yourself!invoked
  • Game-Over Man: Under certain circumstances, the Game-Over Man is different from the usual:
    • Being defeated by the Neutral Final Boss causes him to taunt you during the Game Over screen after Asgore says his usual line.
    • During the No Mercy Final Boss, trying to spare the boss at a specific point will cause him to kill you. The resulting game over screen will have Dogsong play instead of the regular Game Over tune while the boss will assert that, after all of your genocidal actions, he refuses to allow the battle to resolve peacefully.
  • Gameplay and Story Integration:
    • Getting a combination of endings affects the outcome of another. In the Genocide ending, the Fallen Child kills everything and decides to move on to another world... the reset world of Undertale, which would only reset if you took a Deal with the Devil with them. Doing a Pacifist run afterwards heavily implies the Fallen Child's still in control, has taken down every main character, and is moving on to the surface world... and other worlds.
    • On the route for the True Ending, killing any monsters from the True Lab onwards, including the True Final Boss, will ruin the point of it being a Pacifist run. The game still lets you try, however. Attacks will just miss, partially because you wouldn't have the will to kill anybody at this point, partially because said monsters are either phantom-like in nature or a physical half-god(ess).
    • To a smaller extent, conversing with Undyne on her date reveals that she'll probably never let Papyrus into the Royal Guard, but she assures it's not because he's physically incapable, in fact, he's "actually pretty freaking tough"! It happens to be true; by checking their stats in-game prior to patch 1.001, it shows that Papyrus has 8 ATK and 2 DEF, while Undyne has 7 ATK and 0 DEF. Following patch 1.001, Papyrus's ATK and DEF are both 20. Undyne's stats by comparison are now 50 ATK and 20 DEF, but still read 7 ATK and 0 DEF in the code.
    • If you chose to kill Undyne during a Non-Genocide run, she will continue attacking for a few rounds after her HP run out, but her attacks will get progressively slower, while sad music begins to play in the background.
  • Gameplay and Story Segregation:
    • The boss fight against Mettaton EX has an alternate Guide Dang It! pacifist win condition which involves very quickly getting the ratings above 12000. Even if you managed to win the fight fast enough so that Mettaton EX's limbs are still on his body, his overworld sprite still portrays him without limbs during the cutscene that follows.
    • At various points throughout the game, the player is given the option to FIGHT a given character during a cutscene. If the player chooses that option, the basic slashing animation will be used for the attack regardless of whatever weapon you actually equipped. At the very end of a No Mercy run, the Fallen Child will also make the decision for you.
    • There's one point where Flowey doesn't react the way you would expect him to after a reset, which makes meeting him after the reset make less sense. If you abandon a Genocide run (by resetting, since you can't get back onto the Neutral path by that point) after he realizes you're going to kill him, or even moreso if you take the absolute last chance and exit the game before you cut him down in the ending, he doesn't have any unique dialogue about his brush with death, instead recycling his generic dialogue for abandoning the Genocide route where he's angry at you for stopping. On its own, this isn't that bad, but given the heavy Gameplay and Story Integration and massive amount of Developers' Foresight, the absence of a response to that scenario is very noticeable.
  • Gameplay Roulette: While the game is mostly presented as a JRPG, each enemy you battle in Undertale has at least one unique bullet hell-style attack that you must defend against, and each one has a unique text-based "puzzle" you must solve in order to spare them. In addition to this, the Underground itself is absolutely covered in puzzles that must be solved in order to get from place to place, and the game tends to take on horror-like undertones at the end of each route. There are also short mock-dating sim segments that the player can do with a few of the characters.
  • Gender-Neutral Writing: The main character, Napstablook, Monster Kid, and the Fallen Child are only ever referred to using gender-neutral pronouns.note 
  • Genre Deconstruction: Although the game serves as a deconstruction of choice-driven video games as a whole, it puts specific scrutiny on RPGs, breaking down and shedding new light on some of the most fundamental mechanics in the genre.
  • Genre Roulette: Most routes of the game are primarily an adventure/comedy, but with more than their fair share of tragedy and horror thrown in, often with very little warning. Take the genocide route, however, and the game becomes so relentless and unapologetic in these latter elements that it's barely recognizable as the same game anymore.
  • The Ghost: A recurring Easter Egg in certain routes (depending on the game's internal "Fun" value) has the Clam Girl NPC repeatedly reference someone named Suzy, who is allegedly their neighbor's daughter; however, said character never actually appears in-game. If the Nintendo Switch version of the Easter Egg is any indication, that may well be for good reason.
  • Gilded Cage:
    • Toriel's home is a lovely place to live in, but due to her extremely protective nature, she doesn't want you to leave, opting for dodging the topic, excusing herself to leave, and pleading with you. When all else fails, she physically stands between you and the only passage out of the ruins.
    • The Underground in general is a slightly larger cage. The monsters seem to have little to no trouble with infighting or providing for themselves, but they're still living in a fairly small area with no view of the sun or stars. The populace has a widespread sense of hopelessness, both for living meaningful lives underground or ever reaching the surface.
  • Given Name Reveal: The player character's name is actually Frisk, and the character you named at the beginning is really the Fallen Child. Also, Flowey's true name is Asriel Dreemurr, making it clear he's Asgore's lost son and that he Came Back Wrong after his death.
  • The Gloves Come Off: The final battle against Sans at the end of a Genocide run. The game makes it pretty clear that it is done fucking around.
  • Glowing Flora: A lot of places in Waterfall (and a secret location in Snowdin) contain bioluminescent mushrooms that emanate a light blue glow. They can be turned off and on when interacted with.
  • Golden Ending:
    • The Pacifist route ends with everyone down to the last random battle alive and happy, all the bosses becoming friends with each other, and the villains (such as they are) acknowledging their mistakes and prepared to seek redemption if they didn't earn it already. And the Playable Epilogue shows how happy all the monsters are with their new lives. The only dark patches are that Asriel is ultimately resigned to the fate of turning into Flowey once more, and nothing prevents YOU from resetting the save file and erasing all that happiness, though the game actively discourages you from doing so (ironically via Flowey himself begging you not to do it)note .
    • Somewhat interestingly, there's a Golden ending within the Golden ending, if you ACT with monsters in such a way that they're happy with you when you spare them (and monsters you never encounter aren't shown at all), the text in the credits roll will show how successful they were at integrating into human society, with white text being "doing alright" and yellow meaning "found a perfect calling".
    • This is parodied with one of the neutral endings. Sure, you killed everybody who was important in the underground, but having a lazy dog as a president has led to the best life possible for everybody. Thanks.
  • Go Look at the Distraction: During a neutral/pacifist run battle with Mettaton, Alphys instructs you to turn Mettaton around so you can flip the switch on his back, and you do so by telling him there's a mirror behind him. If you lose to Mettaton and return, you can opt to skip his speech and turn him around straight away by telling him there's "something cool" behind him.
  • Good Feels Good: Whenever the game isn't judging and/or guilt tripping you for being a jerk, it's instead encouraging, supporting and appreciating you for being good, or even just not being bad, and it's very good at making you care about your choices.
  • Go Out with a Smile:
    • If you resolve the Boss Battle with Toriel by killing her, she'll acknowledge your strength. Whether that smile is sincere depends on the circumstances when you strike the lethal blow.
    • Undyne The Undying on a Genocide Run.
      Undyne The Undying:(...) This world will live on...!
    • Mettaton, if you get the Ratings past 10,000. It's only temporary, though... he's just out of battery life.
    • Mettaton also does this if you kill him for real on the neutral run, because even if he didn't get to be a star on the surface, at least he got to perform for one human.
    • Mettaton does this a lot. If you play a No Mercy route up until him, but flub the requirements on his boss battle (because you didn't kill everything in his zone before encountering him), he'll say that he can tell that you struck him with less than absolute hatred, and that he can therefore die peacefully knowing that while you might kill Asgore, humanity and Alphys are safe.
    • Both Papyrus and Sans keep their permanent skeletal grins as you kill them in the genocide route; Papyrus because he still believes you can be a better person, and Sans because he's reached the end of his emotional rope.
    • If you kill Flowey after your boss fight with him, he'll flash you a triumphant Slasher Smile and tell you that he "knew you had it in you!" It's very effective at making the bile rise in your throat.
    • If you choose to kill Asgore, or spare him on a second neutral run after killing Flowey (where he commits suicide), he dies with a clearly forced smile, as he's finally relieved that his death will atone for everything he's done.
  • Gray and Gray Morality: The Neutral routes. On one hand, the Player Character is trapped in a fairly hostile world where most monsters are out for their blood, even if the monsters themselves are well intentioned extremists trying to secure their own freedom from the Underground, or attack due to animal instincts. On the other hand, you yourself can choose to respond to this hostility in a ruthless manner by using lethal force. At best, this only results in you causing the death of the Monster King Asgore, as he must be fought in order to finish the game, and possibly Flowey too, after you fight him. At worst, you've left a massive trail of bodies behind you and thrown the kingdom of monsters into despair and disarray. Most new players will likely end somewhere in-between on their first run, making them Anti Heroes in a world where Everything Trying to Kill You.
  • Great Offscreen War: The backstory of the game had a war between monsters and humans that resulted in the monsters being sealed away in the underground. Even though monsters are said to be almost helpless in the face of a sufficiently powerful Killing Intent, humans feared monster-kind's ability to absorb the power of human souls and grow stronger.
  • "Groundhog Day" Loop:
    • Flowey mentions at several points that before you showed up, he used his ability to SAVE to toy with all the characters this way, altering reality so that all of them are technically in one without even knowing it. Assuming that it worked the same way it does when you SAVE, none of them have any direct memories of previous versions of the timeline, but do have foggy recollections of them similar to deja vu or half-forgotten dreams.
    • On a more meta level, because every subsequent playthrough on the same save file canonically takes place in the same in-game universe, you can perpetuate this cycle yourself by playing the game and then resetting. Once again, with the exception of Flowey, none of the characters remember this directly, but do have enough awareness of it to alter their dialogue in several places.
    • Both these examples have a significant curveball thrown into them: Sans. Sans is aware of the fact that he exists in a universe that can be altered and reset at any given moment, and how significantly it affects his character seems to vary based on player actions and Alternate Character Interpretation. At the height of his despair at the end of a Genocide route, he tells you that living with his knowledge has left him in a state of existentially-driven apathy and depression. He then goes on to say that even getting the Golden Ending and making it to the surface world doesn't appeal to him anymore, because it would just look like a setup for a Hope Spot in his eyes; the player is inevitably going to reset sometime anyway, right? As a result, the only way to truly free him from this loop is to never play the save file again after getting the best ending, a move that the game actively encourages you towards and many players have admitted to doing, some going so far as to preserving the file in a thumb drive just to be safe.
  • Guide Dang It!:
    • Figuring out how to defeat the bosses non-lethally in many cases involves paying close attention to clues from other NPCs and to how the boss reacts to different actions. The only way to spare Mettaton EX, for instance, is to either get your ratings above 12,000, or survive until his limbs fall off and get your ratings above 10,000. You're never given any obvious hint for this, and new players might assume the ratings were just for show.
    • Sparing Undyne - the game doesn't really give much of a hint that you even can run from her, or that if you do, that will allow you to spare her. Typically, you don't run from bosses in these kinds of games - only a couple actually let you do that.
    • Following the rule of "Never use 'FIGHT', do the major sidequests" will get you most of the way to the Golden Ending, but this has one possible exception: said ending cannot be the first one gotten, because one of the required sidequests doesn't unlock until you've seen another Neutral ending (if you've been pacifistic for the whole game, you can reload your last save instead of restarting). As a result, you must go against Asgore for a Neutral Ending and use the 'FIGHT' command, although thankfully you can still spare him after fighting.
    • There's a way to get into Sans's bedroom. Once you reach the Last Corridor, repeatedly talk to him and reload your game until he gives you the key. It does make sense, but only in retrospect — who would think to do something like that if they didn't already know the game acknowledged it?
    • In the Pacifist epilogue, you can find Asriel all the way back at the patch of golden flowers in the Ruins where the game started, where he gives backstory about Chara and realizes they weren't as good as he remembered. The Ruins are inaccessible until the epilogue, and he only appears before you see the credits, since otherwise he'll have turned back into a flower.
    • Getting the yellow flavor text for all the monsters in the Pacifist Ending credits. Most of them aren't so bad since they can be gotten either by experimenting or are required to spare the monster without having to fight. However, there are a few of them that are pretty non-intuitive. Some particular examples include Ice Cap, which you can spare by ignoring it two times but to get the yellow credits you need to then steal its hat and then compliment them, and Aaron, which requires you to change the background music in Napstablook's house and then encounter him outside with that background music still on to get his yellow credits, the only enemy in the entire game to do so in events outside of battle. Mad Dummy also stymied the community for a while, as you have to talk to them exactly once during the battle, and there's no obvious connection between this action and either form of their credits text.
    • Arguably, the way to achieve the worst ending (i.e. No Mercy / Genocide route). While the game guides the player quite clearly to the best ending (Flowey always tells you what you missed at any neutral ending you get, unless you've killed him), even the existence of such a worst ending is not clearly hinted, nor the way there instructed. This is deliberate, because it means that there is nearly no chance a player will end up on the route accidentally; a major theme of the route is that the player themselves is choosing to kill absolutely every monster (despite it being unnecessary and tedious) because they want to know what happens if they do.
    • The Kickstarter reward monsters, except for Muffet, are extremely difficult to actually see in-game. So Sorry is found in a hidden room that the player has no reason to believe exists and only appears on one day of the year at an extremely specific time, down to the exact minute. Glyde doesn't appear in a hidden room and has no real-time requirement, but the room it can be found in is out of the way and very small. Additionally, Glyde is an ordinary enemy and not a boss like Muffet or So Sorry, and will only show up if the player walks around in said room for an unreasonable amount of time. To top things off, once the player spares or kills Papyrus, Glyde will never appear again.

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