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Undertale / Tropes Q to Z

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Tropes A-G here.
Tropes H-P here.

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

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    R 
  • Random Encounters: The protagonist shows a "!" speech bubble just before an encounter starts. Initiate the Genocide run, and the speech bubble changes to a smiley face similar to Flowey's and nobody shows up in the encounter.
  • Random Event: The "fun" value in your save file is set to a number between 0 and 100 when you start a new game and depending on what it is, there's a usually a 50% chance that you'll see an event corresponding to that number in the location where it can occur.
  • Readings Are Off the Scale:
    • During your hangout/date with Papyrus, the final stretch of your "Friendship/Dating Power" bar just keeps going right through the bounding box until the screen fades to white.
    • After the January 2016 patch, Temmie no longer has ATK and DEF listed, instead reading "RATED TEM OUTTA TEM". On a related note, Mad Dummy's stats also include "DEF: YES"
  • Reality Ensues:
    • In typical RPG fashion, you might try to sell your unneeded items to the various shops in the world. Unfortunately for you, the only shops you can interact with are food stores that have no reason to buy stuff from customers or people just as eager to get junk out of their inventory as you are. To sell items, you specifically have to find an impulse buyer who's bad at managing their money.
    This isn't a pawn shop, kid. If I spent my money on sticks and used bandages I'd be broke by now.
    • In response to the player killing everything in No Mercy, all the towns are evacuated. You can rob the shops, but all of them give low payouts except for Snowdin, as its residents had to leave in a hurry. The only NPCs around to talk to you are either in situations where you can't kill them and/or ones that are naive about the situation.
    • The No Mercy Run is this from a more meta perspective. In most RPGs, playing as a Villain Protagonist is played for sociopathic Black Comedy, Rule of Cool, or still placing you as a "lesser of two evils" compared to the Big Bad. Not Undertale. A No Mercy Run requires you to actively seek out and kill as many targets as possible and repeatedly makes you feel like an absolute monster for it. All the game's humour vanishes and is replaced by a dreary ambience, NPCs treat you like the scum you are, and every battle is either completely one-sided cold-blooded murder or a rage-quittingly frustrating hard encounter so that you don't even get to enjoy yourself in battles. And because you, the player, chose to be a monster in a game where you don't have to kill a single thing, the game permanently taints all your future save files so you can't just "reset" your way out of the consequences. note 
      • Even the potted plants judge you. On account that it is dead too, from nobody watering it.
    The potted plant is judging you for your sins.
    • As Undyne finds out if you take the pacifist route through her boss battle, chasing someone through Lethal Lava Land as a fish monster who needs water and cold to survive while wearing full plate mail just makes you overheat.
    • Toriel averts the hell out of Easily Forgiven on Asgore. She calls him out on his plan on harvesting human souls, and then again after he backed out of it and prolonged the suffering of the Underground. What makes this particularly jarring is that this attitude is only displayed in the game's Golden Ending where the two interact, aka the ending where literally everything is happy and positive, and that the majority of the remainder of the game's cast agrees that there's still some chance of reconciliation despite this rather cold moment. Then again, Asgore asks if they can be friends, and her response is an extremely exasperated "no," so.
    • Muffet's consumption of spiders seems gross but you have to remember that in real life, spiders are actually cannibals. Specifically, female spiders are more likely to eat smaller males which explains why the spiders in Undertale are so nonchalant about this.
  • Recurring Riff: Several. Flowey's theme and the main theme in particular. There's also a No Mercy route exclusive one used for 2 of that route's 3 exclusive bosses.
    • The sixteen-note pattern at the beginning of 'Ruins' forms the basis for the background music of every area except Snowden.
  • Recycled In Space: The game doesn't shy away from the fact that it's more or less MOTHER made for the modern era.
  • Recycled Soundtrack: A number of songs in the game were composed for earlier projects, such as "Megalovania" (which was previously used in The Halloween Hack and Homestuck).
  • Red Herring:
    • The game heavily misleads you as to how you're supposed to spare Toriel, to maximize the chance that you'll accidentally kill her, then reload so you can spare her properly.note  The scene that results if you do is one of the first major Wham Lines, but it wouldn't have the same impact if you had been Railroaded into it, and the number of Lets Players that have done just that shows that it works.
    • A music-based one. The soundtrack has a song called "Song that Might Play When You Fight Sans". It doesn't. Instead, when you face down Sans at the end of a No Mercy Run... you get Megalovania.
  • Red Oni, Blue Oni: Papyrus is the red to Sans's blue. In terms of Frisk and the Fallen Child, Frisk is the blue to the Fallen's red.
  • Reduced to Dust: Here, monsters don't leave corpses when they die. Instead their body disintegrates into dust.
  • Released to Elsewhere: In Neutral endings where Papyrus is alive but all the other major characters note  are dead, Papyrus says Sans has explained their absence to him by claiming that they have all "gone on vacation". Sans apparently didn't have the heart to him tell him that they were dead.
  • Replay Value: The game has many paths and many easter eggs that occur if you replay it. At the same time, once you get the True Pacifist ending... Flowey will actively dissuade you from replaying it so as to not ruin the happy ending you have created for the characters for your own sick amusement. Of course, maybe you just want to go for True Pacifist again (which is kind of Heartwarming in a twisted way), but the message is probably targeted primarily at the many players who are at this point undoubtedly tempted to go for a Genocide run...
  • The Reveal: At the end of the Pacifist path, you find out that "Fallen Child" — the character you name at the beginning of the game — wasn't the protagonist (they have their own name, Frisk), but rather the "first child", Asriel's best friend.
  • Rewarding Inactivity: In some rare cases, it's better to just stand still to avoid enemies' bullet patterns. Blue attacks enforce this: they won't hurt you as long as you don't move when they are passing. Orange attacks invert it — they won't hurt you as long as you are moving as they pass.
  • Rewatch Bonus:
    • The goat-like monster and child seen within the intro may seem like random characters at first, until you realize they resemble Asriel and the Fallen Child, respectively.
    • The intro sequence almost gives away, to the observant, the fact that the child shown isn't the main character, but rather, the First Child: The main character's shirt bears two stripes, but the First Child's shirt has only one, as shown in the intro.
      • Also, note that the area where the character falls. No flowers. What's the first thing you notice when you play the game? The bright, golden flowers.
    • The circled passage in Toriel's diary is a skeleton-related pun.
    • The cameras you can find here and there are probably Dr. Alphys's surveillance.
    • In Toriel's home, the three rooms on the right are a child's bedroom, Toriel's bedroom, and a third room under renovations. When you get to Asgore's home, the child's bedroom is available but Toriel's is under renovations and Asgore's room is open instead.
    • The tune from the end of the program Papyrus was watching in his house is Mettaton's leitmotif.
    • At the randomized puzzle that Papyrus tries to make you do, the control panel is Mettaton.
    • If you read the lyrics to Mettaton's Opera before he starts talking about dumping you into the dungeon, the lyrics are his honest-to-god feelings on humanity, wanting to keep you away from Asgore because he doesn't want the monsters to break out and kill them.
    • Several, if not all, of the "dirty jokes" you tell Woshua are actually allusions to the backstory of Asriel and the Fallen Child.
    • In the Genocide route, Sans makes a seemingly nonsensical comment apologizing to an "old lady" about breaking a promise. It is only revealed if you spared Papyrus in another run that Toriel made Sans promise to protect any human who came from the ruins.
    • Napstablook mentions that their snail farm used to have a regular customer, but she moved away and now it's just some hairy guy that turns up every so often. An observant player may already have deduced that the regular customer who moved away was Toriel (who makes snail pies), but only in the light of The Reveal that Asgore is a large hairy monster and also Toriel's husband does it become clear that he's been visiting the snail farm periodically because it reminds him of his wife.
    • Papyrus can mention getting toys from Santa. When first playing the game, it's easy to write that off as a joke about Papyrus's naivety, and it's only a long time later that the player has the option to discover a Santa Claus outfit in Asgore's closet, implying Papyrus's toys actually came from him.
    • If you already know the twist in Hotland that Alphys has been arranging all Mettaton's deathtraps so she can play the hero by "rescuing" you, it becomes obvious at several points that she's forgetting or flubbing her pre-scripted lines and Mettaton is covering for her. Further, it's apparent that once you reach the CORE, Mettaton's starting to actually work against her, since the directions she gives you at first are completely wrong, indicating the CORE is no longer configured to her expectations. Also, she's surprised to find monsters there when she isn't expecting any, and the room with the lasers isn't dealt with as easily as previous ones—she has to resort to a Plan B to take care of them, and even then she has to keep fighting to shut the lasers down.
  • Riddle for the Ages: The game has multiple questions that heavily hint towards an answer, but are never outright said, these questions may or may not be solved in the future, though given that Toby Fox is also active in the Homestuck fandom, it was likely deliberate to create Fanfic Fuel and the only true answer being "whatever you decide it is".
    • How long was it between the ancient War and the time the game takes place? Bratty and Catty say it's been millennia and the backstory in Waterfall mentions magicians, but the human shown in the opening looks like a stone-aged tribal. Furthermore, how many years have passed between 201X and when the game actually takes place? This is never even hinted at.
    • What were the Fallen Child's motives for killing themselves? They hated humanity, that's outright stated, but did they kill themselves for Asriel to take the six more SOULs to free the monsters, or did they just want to kill humans? Further, did they actually love their adopted family, or were they just a convenient stepping stone for them? Finally, were they even human to start with, or is their comment at the end of a second No Mercy run about being "the demon that comes when people call its name" meant literally?
    • Just who is Doctor Gaster in relation to the cast? Is he related to the Bone Bros somehow? What was the experiment that scattered him through space-time? Why does it seem like Sans is the only one who remembers him and what is the purpose of that machine?
    • Who were the six SOULs and what were their motivations to go to Mt. Ebott to begin with? All we know about them are the main trait of their specific SOUL and which items they used.
  • Rim Shot: Two of the jokes told by Sans when you first meet him are punctuated this way.
  • Ripple Effect-Proof Memory:
    • The protagonist has one, being able to recall what happened during previous playthroughs. Flowey has one as well. Repeating the opening sequence has him grow more and more angry at you. It also appears that killing him is ripple-proof... until you reach the end of the New Game+. He was just pretending to stay dead to taunt you.
    • In the full version, everyone seems to have it to some extent. Toriel, for instance, will remember whether the protagonist prefers butterscotch or cinnamon. Sans is... a weird case. If anything his memory is weaker than everyone else's, but he supplements it with very good guesswork and being Crazy-Prepared - and he manages to keep a memento from the true ending, after you do a true reset, which is supposed to wipe everything. This is also weaponized by the protagonist in the True Ending's "I Know You're in There Somewhere" Fight Boss Rush after Asriel erases their memories.
  • Room Full of Crazy: In the demo, the manual after No Mercy ending. Almost every page is the same image of a black background with the game's title on it, and red text saying "That was fun. Let's finish the job."
  • Rousseau Was Right: According to Word of God, all of the monsters that the player encounters are basically good and only doing what they think is right, with the exception of Jerry. Even he seems to be mostly an unpleasant but not particularly destructive kind of evil.
  • RPGs Equal Combat: Hahaha, no. The game's tagline is "The Friendly RPG Where No One Has To Die", and one of the first things the game tries to hammer into your head is that you can progress without killing everything in your way; even though monsters will initiate combat with you, they're not necessarily hostile, and you don't have to fight back to get past them. If you try to play the game as that kind of RPG, expect some nasty surprises later on down the line.
  • Ruins for Ruins' Sake: The aptly named Ruins, the first area in the game. It used to be where all the underground residents lived, but they expanded out over time, leaving only a few monsters scattered around.
  • Rule of Seven: The monsters need the power of seven human souls in order to break the barrier keeping them underground. They already have six of them when the player character stumbles into their kingdom...
  • Running Gag:
    • Save points based on mice someday being able to get at the food in the room. In the True Ending's Playable Epilogue, they all do — though the mouse that tried to eat Papyrus' spaghetti ended up giving up.
    • Variations of "Smells like x," where x is something related to the monster you're currently facing.
    • Mettaton's entries:
      Alphys: Oh no.
      Mettaton: OHHHH YES!
    • The antics involving the Annoying Dog. One time, it's preventing you from obtaining an artifact, while another time, it eats the bone that is meant to be for Papyrus's special attack...
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    S 
  • Sacred Hospitality: Comes up if you visit Undyne's house. Much as she'd like to finish you off, she can't, because you're a guest. Eventually defied, as the disastrous "cooking lesson" pushes her over the edge and she attacks you anyway, outright saying she no longer cares if you're her guest.
  • Sadistic Choice: When reaching the end of a neutral run as a pacifist, you find out you must absorb a monster's soul to pass through the barrier. As a result, you are faced with a choice of either resorting to killing or being trapped in the Underground forever.
  • Sand In My Eyes: Papyrus in the Golden Ending.
    Papyrus: I DON'T CRY!! I JUST... CAUGHT SOMETHING IN MY EYE.
    Sans: what did you catch?
    Papyrus: TEARS!!!
  • Save-Game Limits: You only have one save file, and you must reset it to start a new game. This isn't because of engine limitations, but rather because it ties in with the game's theme; your choices are mostly permanent because you can't save to multiple files, and that makes events like Flowey replacing your save file with his own much more impactful. The game itself implies that a human SOUL is what creates the ability to save, thus explaining why you have one save slot and why Photoshop Flowey with six human souls in their possession has six slots. In addition, the implications of resetting the game because the single file is limiting are commented on by several characters, namely Flowey after a True Pacifist ending and Sans during a No Mercy run.
  • Save Point: Appears as a yellow star-shaped icon. The manual describes it as a manifestation of your determination — something in the environment focuses the protagonist's will and enables them to record their progress. In the True Lab, one of the Amalgamations disguises itself in the overworld as a save point (hinted by the fact that it's blocking the way forward).
  • Save Scumming:
    • Discussed. Characters in the game will react if you save and go back to previous events. Going back after killing Toriel and saving her reveals Flowey is able to do this as well, or was able to until you came along.
    • Your character actually considers telling Toriel they killed her last time if you use the "Talk" option on Toriel after reloading. You wisely think better of it on the basis it could make for an awkward conversation.
    • Used against you during the battle with Photoshop Flowey. If you dodge an attack from him, he'll immediately reload a save-state from before the attack (File 2) and hit you with it again. If you succeed in killing him, he'll start into his death-monologue... then reload a save-state from the start of the fight (File 3), bringing him back up to full health, and he saved once again (File 6) just to finish his speech while preventing you from running into the bullets.
  • Schmuck Bait: In Waterfall, the bridge seed puzzle's goal is to cross a body of water to progress, but you can also use the seeds to reach an island with a sign on it. The sign reads "Congratulations! You failed the puzzle!"
  • The Scrappy: Jerry In-Universe: he's rude, ugly, smells bad and whines a lot, you "Spare" him (and in turn the other monsters of him) by out-right ditching him.
  • Screw This, I'm Outta Here!: While you can do this to any monster you're having difficulty befriending without affecting your Pacifist run, you have to do this to "defeat" Undyne peacefully—no amount of talking or stalling will allow you to spare her normally, you just have to flee the battle and keep running.
  • Script Breaking: Prior to the January 2016 patch, it was possible, if you timed it right, to get onto the Golden Ending path despite having killed enemiesnote . The game doesn't actually acknowledge this; if you skip the True Lab and fight Asgore, you don't get the "dirty hacker" ending because the game's logic checks your kill count before it checks whether you've befriended Alphys (so you get the same ending you would have gotten if you hadn't broken the script), and if you do enter the True Lab, the game doesn't check your kill count at all, since you usually have to have a kill count of zero to get there to begin with so that's what it assumes you have.
  • Sdrawkcab Name: Subtle. "Ebott" backwards is "Ttobe", which can be pronounced "Toby", as in Toby Fox.
  • Seasonal Baggage: The different areas of the Underground invokes imagery from different seasons.
    • The Ruins with its fallen orange leaves and ivy plants everywhere is quite autumnal. It is also the first area you are in after taking a fall.
    • Snowdin has — true to its name — snow everywhere. It is very obviously associated with winter.
    • Waterfall is bursting with plant life, and is a underground area full with water, otherwise know as a spring. Alternatively, New Home has flowers and characters that refer to the sun shining and the birds chirping, which can also remind one of spring, especially when considering Waterfall a "non-seasonal" zone for being a cave in a cave.
    • Hotland is very warm and the students that live in the area are on summer vacation.
  • Secret Test of Character: On a meta-level, Undertale as a whole seems to be this for the player. A major running theme is that your actions have lasting consequences even within a fictitious video game world. Even if you reset the game and try again, the game subtly (and not-so-subtly) "remembers" what you did the first time around- whether you went out of your way to spare a monster or whether you just killed it. The implication is that the first thing that the player decides to do reflects their true character, and if you go back and make a different choice after seeing the outcome, you're either trying to hide your mistakes or just trying to see a different outcome for your own amusement.
  • Seemingly Hopeless Boss Fight: The Final Boss of each run.
    • Neutral: In the fight with Omega Flowey, the player can only deal Scratch Damage, the human souls only being able to heal the player from time to time. Eventually, though, the souls render Omega Flowey vulnerable to damage.
    • Pacifist: Asriel Dreemurr completely paralyzes the player upon assuming his second form. It's only after a few rounds of futile struggling that the player gains the option to SAVE some of the souls Asriel had absorbed—and eventually Asriel himself.
    • Genocide: Sans's "special attack" is literally nothing. He tries to make his turn last forever, denying the player any chance to attack. If the player waits long enough, though, Sans will fall asleep, giving the player a chance to move the bullet board over to the "Fight" option and attack.
  • Self-Deprecation: One save point in the Waterfall trash dump describes a very long process involving worthless garbage going down the falls into the abyss as filling you with determination. If you use that save point again, it just says, "Partaking in worthless garbage fills you with determination."
  • Sequelitis: In-universe, with Alphys's online rant about Mew Mew Kissy Cutie 2:
    Alphys: Mew Mew Kissy Cutie 2 Is Neither Kissy Nor Cutie. It's Trash. 0 stars.
  • Series Establishing Moment: When you exit the ruins, Flowey will talk to you and his dialogue varies depending on the actions the player take. Killing anyone will have him taunt you for it, and if you save Toriel but kill any normal enemies, he will also mention that. In the slightly likely chance you kill Toriel and restart to save her, he'll reveal his knowledge of this and your past murder before leaving.
  • Seven Heavenly Virtues: There are seven traits associated with the seven colors you encounter in battle, and each one has a rough analogue to, or shares a name with one of the virtues.
    • Patience is cyan, and shows up in battle on attacks you can only dodge by standing still.
    • Bravery is orange, and is most likely a counterpart to Humility. It shows up in battle on attacks you have to run through in some manner to dodge.
    • Integrity is blue, and is likely Chastity's equivalent. It shows up in the battle against Papyrus and the one against Sans as an effect on your SOUL, turning enemy attacks into a platformer segment.
    • Perseverance is purple, and is Diligence's counterpart. It shows up very rarely, only appearing in the battle against Muffet as a SOUL effect, restricting your ability to dodge by locking it to lines you can jump between.
    • Kindness is green, and shows up in battle as both healing effects, and as a SOUL mode in Undyne's battle, preventing you from moving, but allowing you to block incoming bullets.
    • Justice is yellow, is a counterpart to Charity, and appears in battle the only purely beneficial SOUL mode, allowing you to shoot down bullets in the fight against Mettaton EX.
    • Finally, Determination is red. It is likely Temperance's counterpart, and while it has no concrete in battle mechanic, your SOUL's color defaults to red, and having the associated trait is what allows you to save and load the game.
  • Shall I Repeat That?: Early in the game, Papyrus gives you a set of rules to an absurdly complex tile puzzle, then at the end gives the player the option to hear the explanation again. Subverted in that Papyrus doesn't understand the rules either, and will initially mix up what tile does what in his second explanation before correcting himself. If the player then says that his second explanation confused them even further, he gives up, leaves the instructions on the ground and tells you to read through them on your own and do the puzzle at your own pace once you've figured them out.
  • Shaped Like Itself:
  • Sheathe Your Sword: All battles can be won by "sparing" enemies instead of attacking them, though you may have to perform special actions first; a full Pacifist Run requires this. This is practically its own puzzle during the first Boss Battle: the game starts telling you that talking won't work.
  • She Cleans Up Nicely: Undyne during the fallen human and Alphys' "date." When not fully-armored, she normally wears simple jeans and a tank top with her hair pulled back, but here puts on a stylish jacket, sweater, and lets her hair down. It becomes obvious that she is trying to look good for Alphys. Also applies to Alphys, who trades out her white lab code for a black and white polka-dot dress during the date.
  • Ship Sinking: Toby sunk pretty much every ship that Sans launched when he tweeted that he's too apathetic and lazy for any kind of committed relationship. To be fair, though, it's strongly suggested that he is starting to get better in that regard by the end of the Pacifist Route, and will presumably continue to do so if he lives a good life on the surface, so there may still be hope yet.
  • Ship Tease: In the lead-in to fighting the Pacifist run's Final Boss, Toriel and Sans meet face-to-face for the first time. They share a friendly talk, Toriel's ex-husband Asgore looks more and more bummed out as they proceed, and their sprites hold eye contact as Undyne comforts Asgore with "there are plenty of fish in the sea".
  • Shoo Out the Clowns:
    • An odd case of this happening to the items on the Neutral and Pacifist routes: during the final bosses, any item that had a comedic nature to it will have said comedic element removed (the Butterscotch Pie was shorted to ButtsPie, but in the final boss fights, it will be simply Pie; the instant noodles will be eaten dry and fast (they actually restore far more HP when eaten in this way); and the "Hot Dog?"/"Hot Cat" items will both be referred to as Hot Dogs and won't make an animal noise if you eat them). This also happens during the fight with Toriel. Also, you lose phone connection and can no longer contact anyone once you go into the Core and only get it back in the closing cutscene of the Neutral ending (or if you backtrack to fulfill the criteria for the Pacifist ending).
    • In a No Mercy run, much of the humor in the game is removed, either because you're killing the characters who provide it or because they've ran off to avoid being killed. This extends to the items, where what happens during the most serious fights happens for the entire game. Some other moments are edited to make them less funny, like the Temmie shopkeeper conning you with an expensive Temmie Flake instead of letting you pay for college. There are a few jokes left, but they're either dark humor given the situation or made at the player's expense (including the final boss' jokes about about how frustrated you look after multiple deaths).
    • Also on No Mercy, you need to do this in Snowdin by killing Snowdrake; failure to kill him before you trigger "But nobody came" mode will result in a special save point message and end the No Mercy run. Most players doing a No Mercy run don't notice this, however, due to killing every monster they encounter on sight, including Snowdrake, who will inevitably appear as a random encounter.
  • Shoot the Shaggy Dog:
    • Asriel did this rather than kill humans during the Fallen Child's Thanatos Gambit. He asserted control of his body rather than let the Fallen Child kill the humans, resulting in him being fatally injured by their attacks.
    • Asgore will die at the end of a Neutral Run either thanks to Flowey, Frisk, or to his own guilt about the children he has killed. Averted in a Pacifist Run, where Toriel, who hates Asgore, ironically saves him by interrupting the fight between you and him.
    • Toriel and Papyrus during a Genocide Run. One finds herself betrayed by the child she rescued, and the other is killed by a human he tries to spare, at the risk of losing his potential Royal Guard position.
    • The ending of a Soulless Pacifist run (a True Pacifist run completed after a Genocide Run) is unexpectedly dark. If you choose to leave Toriel, the photo of the royal family displayed in the end credits will replace Frisk with Chara, and cross out the faces of the family members, implying that Chara killed them at some point after Frisk left. On the other hand, if Frisk stays with Toriel, they become Chara, with the implication that they will kill the royal family anyway.
  • Short-Distance Phone Call: If you call an onscreen NPC, you can see their mouth moving during their lines.
  • Significant Anagram: The title of the game itself is an anagram for "Delta Rune", the name of the Arc Symbol of the game. Also, the odd spelling of Asgore's surname ("Dreemurr") was probably chosen deliberately to be an anagram of "murderer".
  • Silliness Switch: Hard Mode. Among other things, several enemies that are normally encountered only in the Core are encountered in the Ruins, with the game lampshading their extremely premature appearance ("That doesn't seem correct."), the flavor text for the Monster Candy pedestal states that you can only take 3 pieces of candy in this "hellish world", and the ending consists of the Annoying Dog announcing the end of Hard Mode after the Toriel fight, Toriel brushing off her defeat (even if you depleted her lifebar) to go bake another pie, a game logo with text announcing that a full Hard Mode is "coming soon...maybe...eh, don't count on it," and Flowey calling you a pathetic tryhard before being cut off by the Dog while attempting to question if you have anything better to do.
    • Inverted in certain fights where the game goes into a "serious" mode, that removes any quirky adjectives in a fight.
  • Similar Item Confusion: A backstory snippet mentions how King Asgore got an accidental food poisoning when his children confused "cups of butter" in a pie recipe for him with "buttercups" (a mildly poisonous flower).
  • Sin Invites Possession: Taking the No Mercy path and deliberately killing everyone possible leads Chara, now a demonic entity, to possess you. This leads to a couple of cutscenes where they move you on their own, as well as the bad ending, where you have to agree to sell your soul to them if you want to play the game again. (Unless you manually reset it yourself.)
  • Sliding Scale of Gameplay and Story Integration: About as far on the integration side as it is technically possible for a game to be. Literally every interaction to be had with the game, even something as mundane as closing and re-opening it, has some kind of in-universe significance, and the ability to save and load progress is one of the most integral driving forces to the game's plot.
    • The concept of gaining experience points and levels is deconstructed toward the end of the game, with Sans having the ability to read your stats and judge your actions accordingly.
    • If you reset the game, or even close and re-open it without saving, it becomes increasingly obvious that the reset isn't an 'alternate timeline' to the one you last played - it's still the exact same timeline, with the exact same world and the exact same characters, being restarted over and over and over again. Flowey has the ability to see through these timelines, commenting on your actions accordingly, and while he doesn't have the same level of awareness, this repetition is a very important part of Sans's character.
    • Infamously, if you bring the game's Kill ’Em All route to completion, the game will end with the world being completely destroyed through a joint effort by you and the Fallen Child (or just the Fallen Child, if you refuse). This will replace the game with an empty black void, and the only way to play the game again is to give your soul to the Fallen Child. From this point onward the game's Golden Ending will be permanently altered so that the Fallen Child takes control of your body at a critical point and implicitly kills all of the friends you've made, with the lasting message being 'you cannot hide from the consequences of your actions'.
  • Sliding Scale of Idealism vs. Cynicism: Depending on how you play, this could be one of the most light-hearted and idealistic games of all time, or one of the most cynical and horrific games of all time.
  • Slippy-Slidey Ice World: The Snowdin region, which is covered in snow and ice. It consists of a giant snowy forest and a cozy winter town. Yes, there's slippery ice as well, but it's used sparingly.
  • The Power of Hate: The first fallen human implies at the end of a Genocide route that the player's sheer violence and bloodlust against the creatures of the Underground gave their body the power to reanimate. If the outcomes of any True Pacifist routes following your first Genocide are anything to go by, this certainly seems to be the case...
  • The Social Darwinist: "Kill or be killed" is stated to be the guiding philosophy underground. Whether anyone actually believes that except for Flowey (and, optionally, the player) is another matter entirely.
  • Society Marches On: In-Universe; Many years ago, humans declared war on monsters (while what started it isn't explicit, some tablets imply it was out of fear of the monsters' ability to gain immense power by absorbing human souls, while humans are unable to absorb monster souls). On the flip side, Word of God is that the happy ending at the end of a True Pacifist run where every monster is shown integrating seamlessly into society is meant to be taken at face value, meaning humans' opinion of monsters has really changed in that time frame.
  • Solve the Soup Cans: Parodied with Sans's "puzzle," a word search on the ground. Literally nothing forces you to look at it, let alone complete it. (It's also technically unsolvable because of a typo.)
  • So Proud of You:
    • Sans tells you this during a True Pacifist run while explaining what EXP and LOVE stand for — Execution Points and Level of Violence — because he knows that you had the capacity to kill monsters, but you didn't, finding a "better" path.
    • Sans tells you this if you abort a Genocide Run by sparing Papyrus.
  • Sorting Algorithm of Evil:
    • Played straight with the random encounters, but averted with the bosses. They mix up the mechanics of dodging enough that the actual damage they do is almost irrelevant to how well you can handle each new system. Also, on a No Mercy Run, only two bosses pose a serious threat to you (one of them being the final "real" fight), which are surrounded by several other encounters that go down in one hit.
    • A fairly unique case in pacifist runs — each successive boss seems designed to successively test your pacifism. It seems fairly intuitive to refuse to use force on a kindly old lady and to try and spare an Ineffectual Sympathetic Villain who seems like a good-natured fellow to boot and will abort the battle before things get deadly. But can you maintain a Thou Shalt Not Kill policy in the face of a Determinator out for your blood, or a well-intentioned but still deadly Killer Robot? And from there: a Monster Lord who has sworn to destroy you against even his own better nature, or even a wholly evil demonic flower that's some sort of soul-devouring abomination? You get the chance to spare them all, even though some are much less likely than others to earn your sympathy.
  • Soul Power: Enemies directly attack your SOUL. Monsters can also gain immense power by absorbing a human SOUL.
  • Soundtrack Dissonance:
    • The song that plays when the bird flies you over a small gap (called "Bird That Carries You Over A Disproportionately Small Gap") is overtly dramatic for the context it appears in.
    • "Unnecessary Tension", an ominous theme that plays only once, while you walk down a completely empty hallway in which nothing happens. And "Dating Tense", a dramatic and fast-paced song that plays during the middle of the Papyrus date when he considers events to be getting intense.
    • Visiting Grillby's during a No Mercy run after you've exhausted Snowdin's kill counter still results in "sans." playing, even though a) the town, and by extension Grillby's, has been evacuated, and b) the track for the town has been slowed down or replaced with the "But nobody came" BGM. This was probably intentional, however, as the track will stop playing once Papyrus has been killed; his death leaves such a significant impact on Sans that everything about him, even his theme song, completely disappears from the game.
  • Source Music:
    • If you stay at Snowed Inn, the neighbors next door snore to the tune of "Determination" (the song heard on the Game Over screen).
    • During Muffet's boss battle, the flavor text between turns occasionally mentions the spiders clapping and dancing to the rhythm of her theme.
  • Space Whale Aesop:
    • The Genocide route is still incredibly horrific and effectively deconstructs the concept of grinding in RPGs and killing characters you otherwise love in subsequent playthroughs just to see what happens, but the very last scene takes it Up to Eleven and turns the message of the playthrough into 'don't go murdering innocent people otherwise you'll get possessed by a creature of pure evil and be forced to destroy the entire world whether you want to or not'. Made worse by the fact said creature possesses your character in every subsequent playthrough and preventing you from ever achieving the Golden Ending ever again, throwing a layer of Fantastic Aesop on top of that by implying 'you can't always use time travel to escape the consequences of your actions'.
    • For the pacifist run: try to resolve issues not with violence, but by talking them out, or else you will be leaving an entire race to be trapped underground because they would not all gather by the end so that the villain could use their souls (and six previous human souls) to have enough power to break the barrier keeping monsters trapped in should you talk him out of throwing reality in an endless reset loop to play with you because he confused you with his adopted sibling.
  • Spikes of Doom: Early on the story, one puzzle is a field of spike panels. Toriel holds your hand through the right path out of concern, but if you go back afterwards, you'll realize the panels are harmless.
  • Splash of Color: A few areas, such as Toriel's and Asgore's bedrooms, are almost completely devoid of color — except for yellow flowers. The pacifist ending reveals that the yellow flowers have quite a bit of plot relevance. Monsters are also normally in black and white in battle with a few exceptions, such as Asgore's red trident and Sans's flashing blue and yellow left eye.
  • Spiritual Successor:
    • The entire game is a very clear homage to the MOTHER series, especially the pacifist route.
    • The No Mercy route could be considered a successor to the Demon Path from Soul Nomad & the World Eaters, both involving a blank slate character seeking power by murdering everyone and everything around them in a manner that's both tragic and horrific, all while being egged on by an evil supporting character.
    • The game can also be seen as a Spiritual Successor to Moon: Remix RPG Adventure. In both games, the player character gets sent to a world filled with bizarre creatures that all have colorful personalities and quirks, they gain LOVE to survive (in Moon, this is absolutely necessary to survive, whereas in Undertale, it's completely optional and it's not even "LOVE" in the first place), they have to deal with a malevolent being who decimates the game world and represents the true horror of playing an RPG stereotypically, i.e. killing everything to gain XP, and both have commentary on the very nature of RPGs themselves.
    • The game shares many thematic elements with Nier, such as Humans Are the Real Monsters, parallel timelines, and replaying the game to gain more context on the world and story. Yoko Taro also expressed a desire for Undertale to be translated into Japanese so he could play it.
  • Squick: In-universe; if you decide to use Junk Food from Bratty and Catty to heal during the Mettaton fight instead of MTT-Brand food from Burgerpants (the former is a lot cheaper and heals enough to be worthwhile on a Pacifist Run), you'll gross out the viewers and lose some ratings, which is a problem if you're trying to spare him.
  • Squishy Wizard: Monsters are made out of magic, which allows them to use magic attacks, but makes them quite fragile compared to the more corporeal humans.
  • Stage Whisper: If you try to back out of hanging out with Undyne when Papyrus has already entered her house, you'll hear Papyrus "shouting a whisper" for you to come back.
  • Star-Crossed Lovers: In Mettaton's opera parody, he sings about a forbidden love between him, a monster, and Frisk, a human.
  • Stating the Simple Solution: Toriel when pointing out Do Wrong, Right to Asgore. Instead of waiting for other monsters in the underground to dispatch six humans, he could have merged with one soul, crossed the barrier, and reaped six more souls. Noteworthy is that monsters (as long as they don't fight) live for much longer than humans do, and Asgore could have waited for six humans to die of natural causes or a massive disaster.
  • Stealth Pun:
    • The very first thing you do is get to Toriel's (get tutorials).
    • One of the dogs is named Dogaressa. "Dogaressa" is the feminine form of "doge" as in the Renaissance-era Venetian head of state.
    • Bratty and Catty are vendors who you can encounter just outside the MTT-hotel, off to one side in a little alleyway. This means that Catty is an alley cat... and Bratty is an alley gator.
    • Near the end of the True Pacifist Route: "But it refused" can refer to both not giving up and the literal re-fusing of the SOUL.
    • During the True Pacifist final boss, determination is what keeps you alive whenever your HP reaches 0. In other words, determination is also de-termination.
    • Sans the skeleton loves to tell jokes and being a comedian. He's a Comic Sans.
    • Mettaton is a ghost possessing a robotic body Alphys built for him. He's, literally, a ghost in the machine.
      • Mettaton EX is a male-identifying robot with a Camp Gay personality and some feminine features. In other words, he's "android-gynous."
    • Unintentional but Asgore does a Mercy Kill. He kills your mercy button.
  • The Stinger:
    • Neutral (Demo): Flowey sends a message at the manual, berating you for any wrongdoings you did.
    • Pacifist (Demo): Sans and Papyrus have a discussion about the game's future. At the manual, Flowey tells you to not get cocky.
    • No Mercy (Demo): A certain someone invites you to "finish the job" once the full game comes out. Not only this screen replaces most of the manual's pages, but on his page Flowey is then depicted as faceless. Those are big hints towards what the No Mercy path entails.
    • Neutral: Flowey asks you to perform a pacifist run or to clear all sidequests to achieve the best ending. By viewing this stinger over and over, he'll refuse what he perceives as your attempts at befriending him, comment on how Papyrus made a fanclub for him and warn you to avoid Sans. He'll then argue that you're just coming to him to exhaust his dialogue and ignore you from that point on.
    • Pacifist: If you decided to live with Toriel, she brings pie to Frisk while they sleep. If not, you see a picture of the cast happy together. Then Flowey begs you to leave them happy and not play the game again.
    • No Mercy: The Fallen Child proposes you sell your soul to them if you want to restart the game. This corrupts the aformentioned stingers of the Pacifist path, as the Fallen will then possess Frisk and kill everyone.
  • Strength Equals Worthiness: The first boss will allow you to leave the Ruins only if you prove you're strong enough to defeat them. Deconstructed in that if you take the boss's HP down to zero they really do die, instead of just yielding the battle and recovering in the next cutscene as is typical for RPGs. You're fighting Toriel, who's seen many children die due to being too weak to survive and doesn't want to repeat this.
  • Strike Me Down with All of Your Hatred: Flowey to the player after you defeat him at the end of a Neutral Run. If you continue to spare him, he'll claim that he's learned nothing, that killing him is the only way to end it, and that if he's allowed to live, he'll come back and kill you and everyone you love. Continue to spare him despite this, and he'll become increasingly bewildered, asking why you're being so nice to him and exclaiming that he can't understand before running off. If you kill him at any point during this scene, he'll give a Slasher Smile, comment "I knew you had it in you.", and fade to a normal flower.
  • Stunned Silence: If you manage to lose the fight with the tutorial dummy by repeatedly missing your attacks, Toriel will be completely baffled, coupled with a Fascinating Eyebrow, before moving on like nothing happened.
  • Stupidity Is the Only Option: Before you encounter Muffet, the path through her lair is littered with spiderwebs that slow your progress before arriving in an area covered in one big web. It's obvious that you're going to get stuck if you keep going, but there's no other way to proceed.
  • Subverted Punchline: An Easter Egg Prank Call from Sans involves the typical "Is your refrigerator running?" gag, the twist being that he's not making a pun out of the word "running" as with the typical punchline of that joke.
    Sans: is your refrigerator running?
    [if you select "yes"]
    Sans: nice. i'll be over to deposit the brewskis.
    [if you select "no"]
    Sans: okay, i'll send over someone to fix it. thanks for letting me know. good communication is important.
  • Suddenly Voiced: In the entire game are two instances were an actual voice is heard. The first is during a Genocide run after your second encounter with Flowey instead of his usual creepy/maniacal laugh, the sound-bite used is a cartoony, high-pitched "That's a wonderful idea!". The second is during a neutral/pacifist run when you flip Mettaton's switch, which transforms him into Mettaton EX. After a flash of light, a sound-bite of him saying "Ohhh yesss!" in a deep, electronic voice is heard.
  • Sunk Cost Fallacy: At the end of a "No Mercy" run, Sans points out that the whole reason you're doing what you're doing isn't out of any sort of moral desire but simply because you can and therefore feel like you have to, imploring you that instead of working so hard for what is quite frankly no net gain you should give up and do pretty much anything else.
  • Superior Species: Monsters can come across as this if you read into the story and world, even though they don't flaunt it. They're made of magic which allows them to freely access it while humans are implied to either need massive amounts of training or have lost the ability entirely since the barrier was erected. Their food instantly turns into energy, leaving no need for bathrooms or excretory functions. The CORE, the power source for the entire Underground, is a geothermal plant that looks and works like something centuries ahead of when the game is implied to take place. And lastly, monster SOULs are explicitly made of positive emotions, so while a human can be either good or evil, monsters are inherently good and being wicked takes a great personal affront (See: Undyne on the worst neutral paths), or they quickly calm down and begin to regret their malicious decisions (See: Asgore), they just don't have it in them to maintain hatred for hatred's sake. On the other hand, as the ancient war between humans and monsters shows, many of these magical powers turn into an Achilles' Heel later on, due to the immense power of a human SOUL by comparison. While their bullet patterns are impressive and the benefits of being magical never seem to run dry, they're all practically Made of Plasticine to a murderous human, they can hardly damage a sociopathic criminal, and unless they can somehow manage to obtain the SOUL of a human despite their Ret Conjuration powers, they have few ways to level the playing field. In this regard, Humans Are Special.
  • Super Window Jump: Papyrus' hilarious exit during his/your "date" with Undyne. If you examine the window once you have control of your character, Undyne mentions it surprised her, not because it's new, but because he normally nails the landing.
    Papyrus: Whoopsy doopsy! I just remembered! I have to go to the bathroom!! You two have fun!!!
    [somersaults out the window]
  • Surprise Creepy:
    • On the surface, Undertale is, as another influential indie game put it, a nice game for cute children. But Flowey will soon give you a hint of the game's true nature. And as you go on you'll find some very disturbing scenes, especially if you don't play nice...
    • The Pacifist run is generally a very sweet and enjoyable experience, if a bit tragic at times, but occasional scenes such as the True Lab and Sans telling you that if it hadn't been for Toriel, he'd have killed you on the spot serve to remind you the kind of experience you could've had if you hadn't chosen to go that way.
      • On that note, if you start off on a Genocide Run, Sans's cryptic remark of "you're gonna have a bad time" and Stealth Hi/Bye will be the first indication that something's gone south.
  • Suspicious Video Game Generosity: In a No Mercy run, killing Mettaton NEO results in enough EXP to jump from LV 15 to 19. You will most likely need the resulting HP boost for the Final Boss.
  • Suspiciously Specific Denial:
    • Toriel calls you to ask if you like butterscotch or cinnamon, then ask if you're okay with cinnamon/butterscotch (whichever you didn't choose), and finally to ask if you have any allergies. She denies she's up to anything.
    • Annoying Dog denying he ate one of Toriel's pies at the end of "Hard Mode".
    • On the Pacifist Route, after the pretend date with Alphys, Papyrus sends her home from training early and wants you to find her at her lab for no apparent reason except he feels you should.
  • Sword and Sorcerer: The monster "Knight Knight" (a large hulking knight) tends to appear alongside the monster "Madjick" (a floating, grinning wizard).

    T 
  • Take That!:
    • Near the end of the Genocide route, Flowey reassures you that at least you're not as bad as the "sickos" who don't have the gall to play the route themselves, but would rather just watch someone do it; a not-so-subtle reference to the countless players who watched Let's Plays of the Genocide route instead of playing one themselves.
    • There's a late-game interaction between Toriel and another character where Toriel takes a friendly jab at people making explicit art of her because of her age, though since the Underground has internet this manages to stay within the realm of the game; odds are that in-universe art of her like that is probably floating around.
  • Talking Is a Free Action: Averted. If you receive a phone call during action, the action continues. This is made clear in Hotland, where Alphys calls you just as you're about to hit the third switch in a sequence of three; the conveyor belt carries you past the switch without you hitting it. It can be seen earlier if Undyne is chasing you. If you have a significant lead over her when Papyrus calls, she'll continue to run after you, then stop at a predetermined point, even if the dialogue box is already up. Calling Undyne later in the same room has her reveal that she saw you were taking a phone call, and stopped to let you finish.
  • Talking the Monster to Death: The standard way of disposing of enemies (and bosses) in a Pacifist route. Reversed in the No Mercy route where Papyrus tries to do this to you.
  • Telephone Song: The "Wrong Number Song" plays to affirm that you've reached the wrong number.
  • Tempting Fate: Alphys explains how she turned Mettaton into a human-hunting robot by mistake and concludes hoping that they won't run into him. Then we hear thumping sounds and guess what happens next?
  • That Came Out Wrong: In Alphys's potential "date" sequence.
    Undyne: Are you two... on a date?!
    Alphys: Umm, yes! I mean, umm, no! I mean... We were, but we were only romantically roleplaying as you!
  • That One Boss: In-Universe, the final boss of the Genocide run tries to invoke this as one final effort to make the player abandon a Genocide run.
  • Theme-and-Variations Soundtrack: Heavily. Several tunes of the game end up being sped up or slowed down reprisals.
    • "Once Upon a Time" and "Memory" show up in a number of other tracks.
    • Napstablook's house theme called "Pathetic House" actually is a much, much slower remix of "Ghost Fight", their fight theme.
    • "Ghost Fight" itself is later remixed into the Mad Dummy's fight theme, "Dummy!", Muffet's fight theme, "Spider Dance" and Mad Mew Mew’s fight theme.
    • Asgore's theme, "ASGORE" has a part from Toriel's boss theme, "Heartache", as well as the Game Over theme, "Determination".
    • Papyrus's boss theme "Bonetrousle" is a reprisal of "Nyeh Heh Heh!", with some added kick to it.
    • Undyne's boss theme during the Pacifist and Neutral routes, "Spear of Justice" is a remix of the theme of Waterfall sped up. Which itself, is a remix of the theme of the Ruins.
  • Theme Music Power-Up: Some characters use a more upbeat remix of their Leitmotif for their boss theme, such as Papyrus. But the Genocide run's final boss is the biggest of them all, as their normally relaxing tone is replaced with Megalovania. Prepare to have one hell of a bad time.
    Game Grumps: Oh! Is that some Megalovania I hear? We are in TROUBLE, boyo!
  • Theme Song Reveal: In Finale, where about halfway through it starts playing a modified version of His Theme, hinting at Flowey's true self.
  • There Are No Therapists: Many major character can be reasonably theorized to be mentally ill or otherwise dealing with serious emotional issues, especially Alphys, Sans, and the Dreemurrs, but nobody ever mentions therapy. Granted, the game does take place over one or two days, so it's possible it's offscreen or the characters in question aren't taking the effort to seek help.
  • There Is No Kill Like Overkill:
    • If you've slain as many monsters as possible, the first boss battle is over very quickly. At a time when the most damage you've done up to that point is around 30, your attack will deal about 22,000. The cheap shot described under I Surrender, Suckers also does obscene damage.
    • At the end of the No Mercy route, the Fallen Child attacks the game world itself for an endless amount of damage, represented by your screen filling up with nines.
    • If you kill Flowey while on the Neutral path, he turns into an ordinary flower instead of outright disappearing. If you kill him during the No Mercy path, however, they strikes so many times, not only does it kill Flowey, it also destroys the flower completely.
  • They Wasted a Perfectly Good Sandwich: Sans treats you to a burger or fries at Grillby's and talks to you about Papyrus and a suspicious flower that whispers things to Papyrus. After that tense moment, you and Sans leave the counter without eating. An NPC remarks that the food is probably cold by now.
  • Think of the Children!: Papyrus likes spikes and other dangerous obstacles and thinks they're good for kids.
    Papyrus: EACH AREA HAS TO HAVE A PRECARIOUS BRIDGE. IT'S MANDATED BY THE GOVERNMENT. OF COURSE, KING FLUFFYBOY WANTS TO UNMANDATE IT SAFER. WHY!!? WON'T HE THINK OF THE CHILDREN!?
  • This Cannot Be!:
    • Mettaton gives a decidedly half-assed version of this speech during your third encounter with him:
      Mettaton: HOW CAN THIS BE, YOU WERE STRONGER THAN I THOUGHT, ETC. WHATEVER.
    • Flowey appears to be aware of how common this is and repeatedly subverts it by saying it just before he laughs in your face, calls you and your friends idiots, and comes back seemingly stronger than ever.
  • This Is Gonna Suck: As put by the intro to the final boss fight on the No Mercy route: "You feel like you're going to have a bad time." You will, due to said boss being the hardest in the game.
  • Throwing Out the Script: Undyne does this just before her boss fight. If you befriend her and call her from the same area, she'll eventually reveal that this is because she forgot the speech she'd memorized.
  • Throw the Dog a Bone: If you perform the True Pacifist route but miss requirements for the Golden Ending, Flowey (if you've spared him) will perform a Heel–Face Turn, rewind your save, and give you hints to meet all the requirements without a penalty.
  • Too Awesome to Use: The slice of cinnamon butterscotch pie Toriel gives you is specified to replenish all HP. When you're at a point where the cheapest foods will replenish most health, it'd be a shame to just waste this. Invoked, considering it has a special effect on Asgore at the other end of the game.
  • To Win Without Fighting: Refusing to kill monsters is necessary to obtain the Pacifist route (and the Golden Ending).
  • Trade Snark: A couple of NPC's in Hotland will share their favorite Mettaton Moment™ with the player.
  • Tragic Bigot: With very few exceptions (most notably, Toriel, Mettaton, and, ironically, Asgore), prejudice against humans is very ingrained into monster culture, which is one of the reasons why almost everyone who is able to identify you as human tries to kill you. Bratty and Catty talk about how excited they are about Asgore wiping humankind out as if it's a movie they're looking forward to, and you need to go through a lot of hell and high water in order to befriend Undyne because of how strongly she holds her grudge. Considering monsterkind's history with humans, and the fact that monsters have been secluded for so long that they barely know anything factual about them anymore (Undyne thinks anime is supposed to be records of human history, for example), it's very difficult to blame them. It helps that, because of how naturally empathetic they are, the power of your friendship alone is all it takes for them to get over this prejudice during the Pacifist run, and they live in harmony with humans forevermore.
  • Tragic Monster: Flowey, literally. As he's a soulless reincarnation of Asriel, product of the experimentation with determination by Alphys and, by virtue of not having a soul, has no sense of morality and is desperate to do anything that let's him feel anything, no matter how twisted and evil.
  • Trailers Always Spoil: The trailer for the Playstation 4 port discreetly shows off Photoshop Flowey's head and tentacle.
  • Train Problem: During his quiz show, Mettaton throws an "easy one" at you, to trip you up. Likely put there to make you notice (if you hadn't already) that Alphys spells out the letter of each answer to the quiz show with her hands.
  • Trash of the Titans: Within Sans's room is a mini tornado. Examining it reveals the following:
    Narration: It appears to be a self-sustaining tornado made of trash.
  • Trauma Inn: Snowed Inn and the MTT Resort heal your HP beyond max if you stay in a room.
  • Treacherous Checkpoint: The game discusses and deconstructs what it means to "save" in a videogame. By the end it is hard to regard any save point as unconditionally good. A few are especially tricky though:
  • Triumphant Reprise:
    • The Core theme is an energetic, upbeat version of the Hotland music.
    • While the soul attack sections of Photoshop Flowey's boss fight are set to distorted versions of Flowey's theme, Finale, which plays when the player can finally retaliate against him, is a much more hopeful remix.
    • Similarly, Hopes and Dreams and SAVE the World is a soaring remix of the melancholy Undertale, with Flowey's theme incorporated as well. And though Hopes and Dreams is pretty triumphant sounding on its own, SAVE the World cranks this up higher.
    • Asriel's theme from the end of the Pacifist run is this to the song the music box plays in Waterfall.
  • Tsundere:
    • Examining a cactus will result in the observation, "Ah, the cactus. Truly the most tsundere of plants." Coming back at the end of the game and this changes to "It's not like this cactus was waiting for you to come back or anything..."
    • The Tsunderplane enemy in Hotland is a tsundere airplane. Its in-battle actions are reminiscent of the typical tsundere behaviors in anime (like turning up its nose or "accidentally" bumping you with its wing).
  • Turn the Other Cheek: On a No Mercy run, this is how Papyrus tries to get you to cease your murdering spree, by simply sparing you and offering nothing but compassion for you. Whether it succeeds or doesn't is up to the player.

    U 
  • Uncommon Time:
    • The tune "Pathetic House", which plays in Napstablook's house, is in 5/4.
    • "Here We Are" is also in this time. "Amalgam" ups the ante by doing away with a key entirely. Both of these are heard when exploring the True Lab, expressing just how unsettling that part of the game is.
  • Unending End Card: While clearing a neutral run sends you back to the title screen after watching an epilogue and a talk with Flowey, completing the Pacifist Run treats you to big the end words and sweet music after its epilogue. If you cleared a genocide run, these words will be blood red and the sweet music will be replaced with ominous music. Closing and reopening the game will have Flowey talk to you and ask you not to reset from the beginning.
  • Unexpected Shmup Level: Alphys can turn your soul yellow, turning it upside down and allowing it to shoot projectiles. This turns Mettaton fights into miniature shoot-em-ups.
  • Unreliable Narrator: The pre-sealing details given by the monsters clashes with the opening narration. The intro claims that the war between humans and monsters was an even-sided war with reasons lost to time that ended with humans victorious after a long battle, whereas the version given in Waterfall paints it as more of a massacre. The Waterfall story also says the barrier was erected by seven mages, but only one mage is seen making the barrier in the intro. Gerson claims the Delta Rune predates written history and symbolizes the monster's escape, but the intro shows the monsters wearing it prior to the sealing.
  • Unscrewed Salt Shaker: Sans tampers with the ketchup bottle to prank you if you accept his offer of ketchup for your food at Grillby's. He's a good enough sport to give you his lunch after yours gets drenched in ketchup thanks to his prank. If you don't accept the initial offer, he just chugs the bottle of ketchup all in one go instead.
  • Unstable Equilibrium: As your LV increases, your attack stat increases and enemies' defenses passively decrease. Thus, as you gain XP and increase LV, you get stronger and stronger until you steamroll almost everything in your path and nothing can stand up to you.
  • Unwinnable by Design: In the demo, selecting anything other than "cheer" or "flirt" after Napstablook cries themself a hat will prevent you from ending the encounter nonviolently. (Yes, even "spare".) You can't even flee afterwards. Fortunately, this doesn't lock you out of the Pacifist Run ending as Napstablook is incorporeal so they don't count as a kill if you reduce their HP to 0. The full version changes this so that the fight is dragged out much longer, but you will still spare them eventually if you try.
  • Unwinnable Joke Game: The word search that Sans lays out in hopes of stopping you (yet can not only be walked past, but still does nothing to stop you if you read it). Every word can be found... except for the gibberish word that looks like the top row of letters but is actually a letter off.
  • Urban Fantasy: The intro gives the year as 201X (although the ending shows the game actually takes place some time later), and despite what one might expect from the setting, things such as cell phones and blogging are quite commonplace in the Underground.

    V 
  • Vague Age:
    • Everyone. No, seriously, everyone; with the exception of one very minor character (Burgerpants, who outright says that he is 19), there isn't a single person in the entire game whose age is ever specified. The protagonist, Monster Kid, the Fallen Human, and Asriel are all clearly children but are never given anything more specific, Toriel and Asgore are on the later end of being middle-aged (though their species don't age normally, so they could be thousands of years old), and literally every other major character could be anywhere from their teens to their thirties and their roles and personalities would still work. And that's not even touching on the fact that at least certain types of monsters age differently than humans.
    • Muffet in particular is the cause for much confusion, as she's described as a "li'l baby spider monster" by her creator, but looks to be in her mid-to-late teens at the youngest, and that fact she lives alone, has a job, and calls the protagonist "dearie" implies that she's least 18. In addition, she lacks the striped shirt that explicitly child-aged characters have. Muffet's designer has gone on record as saying that "li'l baby" was meant to describe her cuteness and not her age, however, so her true age is anyone's guess—although as mentioned, she skews young adult.
    • Papyrus is also an especially bad case, since his personality has a level of naivety and childishness that would be very unusual for anyone older than twelve, but he also has a job, is in training to be a royal guard, lives alone with his brother, and is very tall with the proportions of an adult. This is even lampshaded by Monster Kid if you talk to him after fighting Papyrus:
      Monster Kid: I wonder if that weird skeleton is an adult or a kid.
  • Variable Mix: Several of the game's songs are sped up, slowed down, warped, or otherwise manipulated, sometimes to such a dramatic degree that they sound like entirely different songs. "Ghost Battle", "Dummy!", "Pathetic House", and "Spider Dance" all contain variations on the same tune. "The Choice" is a section of "Undertale" slowed down to 666%, and "But Nobody Came", the theme that plays once you've cleared out all the monsters in a particular area, is a similarly very slow version of "Your Best Friend".
  • The Very Definitely Final Dungeon: In the No Mercy and Neutral routes, The Core is the final area that has random monster encounters before the Final Bosses, in the pacifist ending, the True Labs are this instead.
  • The Victim Must Be Confused: Defied. When the protagonist first meets the two royal guards loyal to Undyne, Undyne will let them know that she had told them beforehand that she'd likely to have been brainwashed if she ever became friends with a human if they contact her after befriending her. This is why she doesn't bother coming to talk them into letting you through.
  • Video Game Caring Potential:
    • To be caring, there's a lot more to it than just being "kind." In order to get the best ending, you have to work your ass off sparing monsters, being nice to everyone, and so on. This takes a lot of work, and one of the points that Flowey makes during the Pacifist run is that you may not be able to keep this up for long.
    • There are two specific instances of this trope that have nothing to do with achieving the Pacifist ending and in fact don't benefit you in any way. One is the snowman near Snowdin who asks you to take a piece of himself with you so he can see the world. Said piece of snow then takes up a valuable slot in your inventory for the entire game (though you can eat it to regain HP) and carrying it to the end of the game simply gets you a remark from Sans that you "made a snowman very happy". The second is giving an umbrella to - of all things - an inanimate statue caught in the rain, which activates a music box playing the Undertale theme, and will net you some lines of dialogue if you call Undyne next to it later on, as well as giving you a hint to a musical puzzle if you stand there for a minute as a small reward.
  • Video Game Cruelty Potential:
    • Discussed and deconstructed to hell and back. One of the major themes of the game is that being "mean" in videogames when you have the option to be kind is nothing short of despicable, and that if you like being cruel instead of kind, you are a bad person. This gets turned Up to Eleven in the finale, when if you've been nothing but cruel, the game labels you as a remorseless murderer who forgoes any chance of sparing those who get in your way. Considering you've been murdering monsters who can easily be convinced to stop or are defending themselves, this can easily ring true.
    • One of the cruelest things you can do is reset the game AFTER you get the true ending. Flowey tells you outright that doing so means undoing all the success and happiness you worked so hard to give everyone, and that choosing to do so would make you even worse than he is.
    • Also one can do a special version of the No Mercy run where you "pretend" Pacifist until the last second when you can spare them and instead insta-kill them. Most special enemies, bosses and mini-bosses have special dialogue for these events where they utterly break down.
    • On the flipside, it is possible to spare many enemies by beating them within an inch of their lives. But if you try this on the bosses, you'll be in for an unpleasant surprise.
    • Even in a No Mercy run, some enemies might still willingly spare you, hoping you are still redeemable. Papyrus being a particularly notable example, though his response if you do spare him after all this time is bordering on Video Game Caring Potential: "YOU DIDN'T DO A VIOLENCE!!!"
    • You're given the option to say "What a loser" instead of "let's be friends" while sparing Papyrus. In addition, you can actually say negative answers during the "date" with Papyrus. This doesn't negatively impact the scenes due to Papyrus Comically Missing the Point each time. You also have the option during a conversation with Sans to say that Papyrus is "not cool" rather than "cool", which will offend him but won't actually change the outcome of whatever route you're on.
    • Even in a Pacifist run, it's possible to still be an asshole even when you're not killing anyone; in addition to saying the mean things to and about other characters mentioned above, you can make the Snowman or the Monster Kid permanently hate you even while getting the Golden Ending:
      • If you eat the Snowman Piece in front of him, he'll refuse to give you another one and then tell you when you talk to him in the Playable Epilogue that, even if you have other characters fooled, he knows what kind of person you truly are.
      • When the Monster Kid falls off a bridge while Undyne approaches you both, the "best" option is to save him from falling, at which point he'll defend you from Undyne, who retreats, and decide that the two of you can still be friends even though you're a human. However, you're not required to save him to achieve the pacifist ending, and can run away and leave him to be rescued by Undyne rather than doing it yourself. This causes him to grow to hate you, and if you interact with him in the Playable Epilogue, he'll tell you not to talk to him.
  • Video Game Cruelty Punishment:
    • "Killing" Napstablook will earn you a negative experience point. Subverted in that this doesn't affect the game in any way. In this game, Experience points are something entirely different from "EXP".
    • This is the only way to describe No Mercy-Run final boss, aka, the hardest boss in the entire game. You only face this rule-breaking Marathon Boss if you kill every single enemy possible, and he'll say anything he can to make you feel bad about it.
    • If you do go through the entire No Mercy path, the Fallen Child will erase the game itself. Pester them about it and they'll call you somebody who believes themselves to be above consequences. You can then start over by selling your soul to the Fallen Child and boy, that doesn't end well at all.
  • Villain Protagonist: The main character on the No Mercy route. On a thematic level, the game discusses the nature of playing this way in video games, and the idea that the player is doing so just to see "what would happen". Going through with a Genocide route play-through also deconstructs the role of the player, with the game destroying itself because you killed everyone in the entire game. The Fallen Child even reacts with abject disgust if you ask for the game world back in exchange for your SOUL, wondering if you believe yourself to be above repercussions for your despicable actions. As it turns out, you most certainly are not- doing so results in the game permanently tainting all further playthroughs to ensure that even on the best ending, everyone will die.
  • Violence Is Not an Option: Downplayed in the Pacifist Ending. The Final Boss is immune to physical attacks, so attempting to fight him the traditional way is never going to end well for the player. However, as this is a Pacifist Run, ending encounters without killing anyone isn't exactly a new concept.
  • Violence Is the Only Option: For a game that promotes pacifism and nonlethal resolutions, your first fight against Asgore makes it clear that you have to take him down the hard way. If you've been playing pacifist up until now, prepare to get schooled, since you're never really allowed any chance to practice how to fight beforehand without it changing your run to Neutral.
  • Visual Pun:
    • Muffet's bake-sale table has eight legs, just like a spider.
    • What looks like an asterisk on the Tem Shop sign could be an "I" crossed out, so it would read "Item Shop".
    • When your SOUL turns blue it can no longer fly. Being blue gives you a heavy heart.
    • If you take too long in Mettaton's maze section, Alphys will help you by hacking the firewall. That is, a literal wall of fire that is approaching you.
    • Photoshop Flowey's hand-cannons have literal green thumbs.
    • In the Nintendo Switch port teaser trailer, Sans and Papyrus are seen standing on a Switch gamepad. Sans is standing on the left side next to a blue Joy-Con, while Papyrus is on the right next to the red Joy-Con, matching their clothing.
  • Voice Grunting: Even though the game does not have any genuine voice acting, every major character makes a different sound when their words are dispensed on the screen. The ability to recognize them by this voice alone is essential for a couple of twists. The only two characters who have genuine voiced dialogue are Flowey and Mettaton, but only one sentence each.

    W 
  • Waiting Puzzle: One option for getting past the force field in the CORE is to just stand in front of it for a few minutes. "I cannot fight. I cannot think. But with patience, I will make my way through".
  • Wakeup Call Boss: Dogamy and Dogaressa are oftentimes the cause of the first death to even more skilled players, due to them being the first enemy that requires use of multiple ACTs to end the battle peacefully (Roll Around, Sniff, Pet), utilizing blue attacks in close proximity to regular attacks, and their axe attack, which covers almost the entire bullet board and requires precise movement to dodge.
  • Walking Spoiler: Literally the perfect definition of this entire game.
    • A notable non-character example is the now-famous track "Megalovania", which spoils the genocide ending.
  • Wall of Text: If you die during Omega/Photoshop Flowey, you'll get the standard Game Over message followed by "HAHAHA" repeating over and over.
  • Watsonian vs. Doylist: The Game thoroughly explores the dilemma behind and between both of these stances and ultimately deconstructs the whole concept.
  • Weaksauce Weakness: Monsters, all of them, are composed of mostly magic, and this makes them very sensitive to negative emotions. Thus, someone with a hateful heart and Killing Intent can disrupt a monster's soul enough to kill them in a single blow, even with something that isn't actually a weapon. However, the amount of intent it takes to get to that point is left ambiguous. The Fallen child was completely genocidal; it's possible a well-adjusted human wouldn't be capable of hating that much, if Monster-kind's easy reintegration in the true ending is any implication.
  • We Buy Anything: Parodied: despite the fact that every shop has a Sell option, selecting it just causes the shopkeeper to mock you, stating that they don't either have any use for your items or that buying your stuff makes no sense from a business POV. The only shopkeeper that's willing to buy your items is a massively ditzy Cloud Cuckoo Lander.
    Snowdin Shopkeeper: Does this look like a pawn shop? I don't know how it works where you come from... but... If I started spending money on old branches and used bandages, I'd be out of business in a jiffy!
    Gerson: I'm trying to get RID of my junk, not get more of it!
    Burgerpants: (WHY IS THIS PERSON TRYING TO SELL ME SOMETHING THIS IS A HAMBURGER RESTAURANT I'M JUST TRYING TO SURVIVE)
  • We Can Rule Together: On a No Mercy run, after clearing the Ruins, Flowey acts like the player character is an old acquaintance of his and asks for assistance in destroying the world.
    Flowey: THAT'S A WONDERFUL IDEA
  • Welcome to Corneria: Papyrus lampshades this when you call him at the Hotland dock:
    Papyrus: HEY, YOU SHOULD COME TO SNOWDIN AND VISIT ME! I'VE BEEN WORKING ON A FEW THINGS. A FEW SENTENCES TO STAND AROUND AND REPEAT.
  • Well-Intentioned Extremist: The majority of monsters only want to kill you so that they can use your SOUL to escape the Underground.
  • We Will Meet Again: If you kill just about every leader and enough monsters to make Sans disgusted with you without going No Mercy, he ends with two very chilling words: "see ya." Kill any more monsters than a certain limit, and he'll tell you to go to hell.
  • Wham Episode: The endgame for any route is bound to be one of these.
    • New Home, Neutral/Pacifist Run. You go through a home that looks almost exactly like Toriel's, and learn of the tragic tale of how King Asgore lost both his adoptive child and his own birth child on the same day. Then you run into none other than Sans, who reveals to you the true meaning behind EXP and LV. And then you meet King Asgore himself, who you're forced to fight against until his last bit of HP. Regardless of whether or not you choose to spare him then, Flowey shows up and kills Asgore, destroys his soul, takes the human souls for himself, and then hijacks your ability to save.
    • New Home, Genocide Route. Having nearly completed your genocide of monsterkind, Flowey shows up and talks with you about his backstory, coming to the terrifying realization in the process that he's not exempt from the carnage. You then run into Sans, who proceeds to show you a bad time as the hardest boss in the game, but ultimately, he too falls and the Player proceeds to finish his Genocide with killing both Asgore and Flowey. With your Genocide complete, the Fallen Child makes their appearance and proceeds to finish the job by destroying the world.
    • The True Laboratory. You learn about Alphys's experiments with Determination, which is an actual tangible force. By injecting dead monsters with Determination, they came back to life, but their bodies couldn't hold themselves together and the eventually fused together into horrifying Amalgamates. Also, it's heavily implied that Alphys created Flowey by injecting a flower with Determination. And all of this leads into...
      • True Pacifist Ending. Flowey manages to take not only the human souls, but the souls of every monster in the underground, and assumes his true form, Asriel Dreemur, Asgore and Toriel's deceased son. And at the end of the final battle, you learn, if you haven't figured it out already, that the child you've been playing as is not the child you named at the beginning of the game. The child you named was the child who Asgore and Toriel adopted, and the real name of the child you've been playing as is Frisk.
  • Wham Line: Hoo boy, where do we start?
    • If you accidentally kill Toriel in battle, reload/restart your game, and face her again:
      Toriel: ...wait. ...why are you looking at me like that? Like you have seen a ghost. Do you know something that I do not? No... That is impossible.
      Narration: You thought of telling Toriel that you saw her die.
    • The first thing you hear from Flowey indicating that he remembers your past playthroughs. Depending on the path you take, there are several pieces of dialogue that could end up being this.
      Flowey: You naive idiot. Do you think you are the only one with that power? The power to reshape the world... Purely by your own determination. The ability to play God! The ability to "SAVE."
    • A character displays knowledge of Flowey: "have you ever heard of a talking flower?" It turns out the person who asks the question, Sans, actually doesn't know about Flowey—he thinks someone's using an echo flower to mess with his brother Papyrus.
    • In a meta sense, if you've been playing this game like a typical RPG, The Reveal during Sans's judgement is going to hit HARD. These lines completely change the way the player thinks about the game.
      Sans: What's EXP? It's an acronym. It stands for "execution points". A way of quantifying the pain you have inflicted on others. When you kill someone, your EXP increases. When you have enough EXP, your LOVE increases. LOVE, too, is an acronym. It stands for "Level of Violence". A way of measuring someone's capacity to hurt.
    • Two from the same character that show you just how much your actions impact the world.
      [from the No Mercy run] Sans: if you keep going the way you are now... you're gonna have a bad time.
      [from the Pacifist run] Sans: do you get what i'm saying? that promise i made to her... you know what would've happened if she hadn't said anything? buddy... You'd be dead where you stand.
    • Near the end of the Pacifist run, when Alphys asks how everyone knew to go to the throne room in the first place.
      Papyrus: LET'S JUST SAY...A TINY FLOWER HELPED ME.
      Alphys: [makes an Oh, Crap! face] A tiny... flower?
    • Pretty much anything the Fallen Child says is this, but special mention goes to their first appearance in blood-red text.
      Fallen Child: Where are the knives.
    • For most of a Neutral/Pacifist run, you think your goal is just to persuade Asgore to let you leave. Then Alphys drops this bomb at the end of Hotland:
      Alphys: I lied to you. A human SOUL isn't strong enough to cross the barrier alone. It takes at least a human soul... And a monster soul. If you want to go home... You'll have to take his soul. You'll have to kill ASGORE. I'm sorry.
    • When fighting Snowdrake's Mom in the True Lab, some responses to actions like Heckle and Laugh can be interpreted as the first signs that Frisk and the narrator/player are actually separate entities, as Frisk refuses to heckle her.
      Narration: You said something like... "You look horrible." "Why are you even alive?" ... what? You didn't say that?
    • Alphys' notes in the True Lab are all shades of disturbing. But one in particular stands out.
      Alphys: I've done it. Using the blueprints, I've extracted it from the human souls. I believe this is what gives their souls the strength to persist after death. The will to keep living. The resolve to change fate. Let's call this power... "Determination."
    • And later on:
      the flower's gone.
    • The last line from the Froggit before you face Asgore in the Pacifist Ending qualifies. You knew the scene was going there, but very, very suddenly, it becomes very clear that this is the last stand, and that one way or another everything is about to change.
      Don't you see? ...You're going to be free.
    • If you haven't done a No Mercy run yet, the second tape in that same area reveals that there's some background to the character you named when you first started the game:
      Okay, (name)note , are you ready?
    • And after the True Final Boss fight, you get a Wham Line that changes the meaning of the above Wham Line and the nature of the protagonist:
      Asriel: You're not actually (name), are you? (name)'s been gone for a long time. Um...what...what IS your name?..."Frisk"? That's...a nice name.
    • If you've started a No Mercy run and been justifying it with self-defense, well, the opening of the fight with Papyrus makes sure you can't use that justification anymore.
      Narration: Papyrus is sparing you.
    • Toriel warned you about the murderous Asgore, and throughout the game so far you've heard about the friendly King Dreemurr. When Undyne first directly confronts you, she reveals they're one and the same.
    • From the end of the No Mercy run: the player has been in control for MOST of it, save for a couple of... unsettling lines, and your character moving by themselves at some points. But that's a given since you're the one able to SAVE, right? You have the ability to do and undo whatever you like. When The Fallen gives you a choice (ERASE or DO NOT), it looks like you'll be able to finally do the right thing and look like the better person... but it's not as easy as that anymore.
      "No? Hmm...how curious. You must have misunderstood. SINCE WHEN WERE YOU THE ONE IN CONTROL?"
    • Though it's common knowledge now, when the game was new, habitual grinders and people just idly killing monsters were given a huge shock when it's shown that you can depopulate an area and bring with it a gigantic mood shift with a blank battle screen and three little words to signify the start of a No Mercy run.
      But nobody came.
    • The opening of the True Final Boss, making it clear who Flowey really is.
      "It's me, your best friend ASRIEL DREEMURR!"
  • Wham Shot:
    • During the battle against Asgore in the Neutral route, the first thing he'll do is destroy the Mercy option, emphasizing even further there is no other option other than FIGHT.
    • The final battle of the True Pacifist route begins with Flowey absorbing every single SOUL in the Underground except for the protagonist's, which he claims will allow him to reach his true form. As he's doing the absorbing and lights are flashing everywhere, you'd think Flowey would turn into a form even more horrifying than his Photoshop form — until the scene suddenly cuts to Asriel Dreemurr standing with his back facing toward you, solidifying The Reveal that Asriel is Flowey's true form.
    • At the end of that fight, you're given the ability to SAVE Asriel, which cuts to a flashback of the intro... then Asriel shows up, revealing that the child who fell in the intro is not the child you actually play as.
    • Three almost back to back at the end of the worst possible run. The first is Sans's first attack, in which he gains a glowing blue eye and demonstrates his incredible power by mercilessly slaughtering you. The second is midway into the fight, when Sans's expression, for the first time in the game, shifts from a goofy grin to a genuinely pained expression. There's also the part where he dodges your attack, the only enemy to ever do so. Finally, when you defeat him, he has blood (or what appears to be blood) coming out of him, something no other monster in the entire game does, even though monsters are explicitly described as being made of magic (resulting in their physical forms collapsing to dust upon death).
    • Players who engage in a lot of random encounters either just to kill enemies for kicks or to level grind eventually get a rude awakening when the random encounter flow begins, only for no enemy to show up, along with the narration text reading "But nobody came."
    • For players that have previously done a very "dusty" Neutral run and fought boss monsters legitimately the first time around, then start a proper No Mercy run, seeing their attack do a One-Hit Kill to Toriel can come as a shock.
    • One could mentally justify the first bits of a No Mercy run with self-defense taken to the point of overkill, since everyone tries to kill you (or fakes it, in Toriel's case) up to that point. But when you reach Papyrus, it becomes clear that he has no intention of even attempting to fight you, instead trying to turn you away from your path with kind words, an offer of friendship, and immediately wanting to let you go. You can't claim self-defense to justify it; proceeding comes at the cost of striking down the friendliest, most harmless character in cold blood while he welcomes you with open arms. This is the first inarguably evil moment of the path, and fittingly tends to cause people who can't handle it to abandon the No Mercy path.
    • On No Mercy, players who killed Toriel and Papyrus and everyone else (except for Jerry) in between with relative ease get a second shock when they attack Undyne the Undying and see their attack only take off a small fraction of her lifebar, instead of instant killing her like the other bosses and her first form.
    • For those who dug really deeply into the files to find the information on Dr. Gaster, patch 1.001 added one of these when people around the internet started posting shots of Gaster's door in Waterfall, or his followers in Hotland, appearing in worlds where the files weren't edited, meaning Gaster's somehow restoring himself without outside influence.
  • What Measure Is a Mook?:
    • Killing any enemies while saving Toriel will result in a massive Player Punch later on.invoked
      Flowey: So you were able to play by your own rules. You spared the life of a single person. [lists off each monster] Think about those names. Do you think those monsters had friends? Do you think they had families? Each one could have been someone else's Toriel. Selfish brat. Someone is dead because of you.
    • The Genocide route features a bizarre inversion. To complete the No Mercy route, you have kill a certain number of monsters in each area by the time you encounter the area's boss, as well as killing all the minibosses and bosses (and Snowdrake). note  However, you can spare as many generic monsters as you want and still complete a No Mercy run as long as you meet your kill quota by the end of each area (and kill all the unique monsters (including Snowdrake, who you can spare as often as you want as long as you eventually kill him)). So, basically, you can let some minor monsters get away while making sure to kill enough to depopulate each area (as well as the major monsters)
  • What the Hell, Player?:
    • Killing any significant character will have this sort of effect come from somebody, so as you can imagine, this becomes the game's constant attitude towards you during a genocide route.
    • Even if you spare every other monster in the game, killing Papyrus causes Sans to completely vanish from the game until the very end, where he doesn't hold anything back as far as letting you know just how much of a heartless decision that was.
      Sans: this is an odd thing to say, but... if you have some sort of special power... isn't it your responsibility to do the right thing?
      Yes
      Sans: ah. i see. ... T h e n w h y ' d y o u k i l l m y b r o t h e r ?

      No
      Sans: heh. well, that's your viewpoint. i won't judge you for it. ... Y o u d i r t y b r o t h e r k i l l e r.
    • Taking too much candy from the candy bowl.
    • The worst endings of the first zone with Toriel and Flowey's speeches also qualify.
    • The ending narration for most of the less pleasant neutral endings will call the player out on their actions.
  • When All Else Fails, Go Right: The starting point is the westernmost point in the game, and you largely move towards the east, with the end of the game being the easternmost point.
  • "Where Are They Now?" Epilogue: The true ending credits display or tell what every character, monster encounters included, is up to after being freed. The neutral endings, meanwhile, have the characters themselves inform you of what happened next. Also, if certain conditions are met during certain monster encounters, when the ending credits roll describing what the monsters are up to now, they are replaced by yellow text doing something else.
  • Where the Hell Is Springfield?: For presumably the same reason as most other ambiguities about the world, we're never told or hinted at where Mt. Ebott is in relation to the rest of the world. The culture the monsters have seems very American and the city we see in the background in the true ending looks decidedly American with the skyscrapers, but the backstory mentioning magicians and way the Underground is run by a monarchy suggest European, especially since it's implied the Monsters would have been sealed at the height of the middle ages.
  • White and Grey Morality: The True Pacifist Route. The only truly villainous characters in this route are the Fallen Child, Flowey, and Asriel. Out of these, the Fallen Child is an Ambiguously Evil human who actually brought a lot of hope to the kingdom of monsters before setting into motion an Evil Plan that resulted in the demise of both them and Asriel. Asriel himself is a Tragic Villain who just wants to be reunited with his lost friend and can't bring himself to move on from what happened. And Flowey is a reincarnation of Asriel, whose vile and sociopathic nature stems from the Lack of Empathy he got saddled with after becoming a flower. Everyone else is a Well-Intentioned Extremist at worst, and in order to succeed in this route, you yourself must go out of your way to portray the Messianic Archetype.
  • Widely Spaced Jail Bars: Papyrus attempts to block your path with a set of these, which Sans points out can't actually stop you. Later on, it becomes evident that Papyrus hasn't learned anything; if he defeats you, he "locks" you in a cell that you can literally walk out of.
  • Worked Shoot: Subverted. During the final battle against Mettaton, he admits that your previous battles with him were an act. However, at this point, he has concluded it's necessary to kill you for real. It's implied the shoot was starting to get un-worked as you entered the CORE.
  • World of Pun: Boy, is it ever. Sans might be the most notable offender, but the puns are everywhere... Yes, even in your menu.
  • Writer on Board: Happens a few times, both with the same character.
    • Near the end of the No Mercy route, Flowey has some choice words for people who watch the run online and are too cowardly to do it themselves.
    • After the final boss of the pacifist run, the player can return to the beginning of the Ruins and talk with a ghostly Asriel. If the player talks to him repeatedly, he'll go on a philosophical tangent about how in the outside world, unrelenting pacifism may not always be a viable option, and that sometimes, the best you can strive for is "don't kill, and don't be killed" (echoing Flowey's catchphrase).
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    Y 
  • Year X: The opening cutscene takes place in "201X." The game itself takes place much later.
  • You Are Better Than You Think You Are: During a Genocide run, Papyrus will try to stave you off your destructive path by assuring you that you're a great person at heart and, if you just try, no matter how little effort you put into it, the great person you are will shine through. Should you kill him anyway, despite his speech, he'll even use his last breath to promise you that he knows you can do better, even if you don't think so yourself. The amount of faith he puts in you has proven to be just as effective as Undyne's and Sans's beatdowns when it comes to making people abort their Genocide runs.
  • You Are Too Late: In any neutral ending where she is alive, Toriel will arrive just after Asgore dies. It doesn't matter how long you take, or if Asgore kills himself, you kill Asgore, or Flowey does the deed. The only run with the subversion is the True Pacifist run, where she shoots Asgore with a fireball and ends your fight with him before it begins. It's implied that the time you spend in the true lab ends up buying you enough time for her to mount a rescue/intervention.
  • You Bastard!:
    • The game repeatedly tries to invoke this against the player for taking violent options. Both implicitly, with plenty of Videogame Caring Potential and Player Punches, and explicitly in Flowey's speeches after the encounter with Toriel.
    • As listed under Take That!, even players who don't actually play the "kill everyone" route aren't safe from being shamed, as Flowey suggests that Undertale owners who are watching a playthrough of the route just to see what it's like are too cowardly to try for themselves, and are arguably worse than those that do for indulging in this version of the story while thinking that they're still above the consequences.
    • Notably, a rare example of a game which invokes this for actions taken in past playthroughs, such as the crime of trying to achieve 100% Completion when doing so requires you to reset the Golden Ending you've achieved and instead murder all the people who were once your friends. The game not only calls you on your belief that you can do these things with no consequences, but is designed to ensure that any attempts to get the Golden Ending afterwards are tainted forever by what you did if you follow that path to the end.
  • You Could Have Used Your Powers for Good: If you kill Papyrus without going full genocide, Sans will confront you just before the final battle and indirectly give you this accusation by phrasing it as a question.
    Sans: sometimes... you act like you know what's gonna happen. like you've already experienced it all before. this is an odd thing to say, but... if you have some sort of special power... isn't it your responsibility to do the right thing?
  • You Mean "Xmas": Snowdin has a tradition of giving presents under a tree, which originated when some pranksters started putting decorations on Gyftrot.
  • Your Size May Vary: Characters' battle sprites aren't drawn to the same scale relative to each other as their overworld ones. It's not too noticeable for most of the game, mostly because major characters tend not to appear in-battle with each other, but during the True Pacifist ending, all of the main characters are shown together both in the battle interface and on the overworld, and there are obvious differences. For example, Asgore is, judging from his overworld sprite, supposed to tower over everyone except Toriel, but in-battle Undyne is almost a full head taller than him.

    Z 
  • Zero-Effort Boss: Most bosses and enemies on a No Mercy run can be killed before they even get to act.
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