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Undertale / Tropes H to P

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You can view the main article here.
Tropes A-G here.
Tropes Q-Z here.

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

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    H 
  • Half the Man He Used to Be:
    • If you kill Toriel on a No Mercy route, she will have visible slices on her body and in her face.
    • In the No Mercy route, Undyne takes a blow for the Monster Kid that visibly cuts her battle sprite in two, from shoulder to hip. Her upper half begins to slowly fall off... until she reforms herself for the real boss battle.
    • The final boss of the No Mercy route has his torso diagonally slit in half all the way from his shoulder to his hip.
  • Hand Wave: Whenever the player is slightly out-of-bounds (when it should be impossible), "magic glass" appears underneath them so they won't be walking on air.
  • Happily Ever After: Everyone in the Golden Ending, except Flowey, and even he's better off than he was at the start. From Hotland's two royal guards to Alphys and Undyne, almost every other monster and their corresponding Mind Hive Long-Lost Relative amalgamation, and possibly Asgore and Toriel.
  • Have We Met?:
    • In the True Pacifist Route, Toriel and Sans who had never met in person find each others' voices familiar and realize that they'd been acquainted for quite some time.
    • When you restart the game, any of the main characters that you had befriended in a previous playthrough will vaguely remember you. For example: when Toriel asks what your favorite is between cinnamon and butterscotch, she will instead take a guess, which will be whatever you picked in the last playthrough.
  • He Knows About Timed Hits: The monsters in the Ruins will gladly tell you all about hitting X to skip conversations, how a monster's name will turn yellow when you can SPARE it, and other helpful information. Of course, this extends well past the tutorial stage, leading up to the revelation that Flowey knows about and controls SAVE FILES in the final boss battle of the neutral route.
  • Healing Checkpoint: They also serve as save points.
  • Health/Damage Asymmetry: Justified: Though many monsters are a lot more powerful than humans, with characters like Undyne that can suplex boulders being fairly common, their genetic make-up causes them to take massive damage when hit by a foe with a lot of Killing Intent. Humans, meanwhile, can use determination to live through magical attacks that would otherwise prove incredibly fatal. Thus, this trope.
  • Heartbeat Soundtrack:
    • A heartbeat plays in background of the final scene of any Genocide playthrough, where the player has killed every single NPC the game allows.
    • In "But the Earth Refused to Die": If you listen closely, you can hear the hearts of everyone in the world beating as one.
  • Hell Is That Noise:
    • There's a very distinct and awful scratchy sound that plays whenever you interact with a Gaster follower immediately before they vanish into thin air, and that's not even getting into the hellish cacophonies used for any of the Dummied Out rooms.
    • If you kill all monsters in an area, Random Encounters still occur, but only display the words "But nobody came", and from there the music for the area is replaced with a demonically slowed down version of Flowey's theme.
    • There's only one fully voiced line in the entire game across all routes. During a conversation with Flowey during the No Mercy run, he mentions wiping out the world. Without appearing in the text box, a horrible voice says "that's a wonderful idea!" Most players will find that one is plenty.
    • Flowey's Evil Laugh never fails to be creepy. It gets ramped up to nightmare-inducing levels once he transforms into his SOUL-powered form. It even makes its way into the soundtrack!
    • The Fallen Child gets two really creepy themes. One is the menacing "In My Way." And the other is a warning drone noise that indicates you've finally met your match.
    • An in-universe example: If you visit Napstablook, listen to one of their music tracks, keep it on when you leave, and go south, you'll encounter Aaron and Woshua. Their reaction is hilarious, and seeing the scene actually helps you get rid of Aaron quickly (and non-violently) if you run into him again.
  • He Who Fights Monsters: A major theme of the game. By continually fighting literal monsters, even to the point of excess, they start becoming actively afraid of you and your capacity for cruelty. Especially apropos because you are fighting creatures called "monsters", yet can possibly end up more horrific than almost anything you encounter if you so will it. It gets even lampshaded by both Sans very early and Asgore very late that the player character can't even be recognized as a human, meaning they visually change into a literal monster as well.
  • Heroic Sacrifice:
    • Toriel will let you kill her to prove that you are strong enough to survive the Underground.
    • Undyne will make a final stand against you in the Genocide Run rather than go into hiding with the survivors, in the hopes of protecting the monsters that haven't been able to flee from you and save the humans.
    • Asriel after his Heel Realization decides to do the right thing and give all the monsters their forms back, at the cost of reverting back to a soulless flower. He also refuses to go with Frisk and his parents to the surface, because he doesn't want anyone to see him turn into a horrible monster.
  • Heroic Second Wind: In the No Mercy route, Undyne takes a fatal blow to protect an innocent from the player. Before she dies, however, her determination triggers a transformation into the powerful Undyne the Undying.
  • Heroic Suicide:
    • Under certain conditions, if you spare Asgore, he starts dreaming about an idyllic future with you, his wife, and him living together as a family. ...But "that's just a fantasy, isn't it?" He tells you about the prophecy of a savior coming into the underground to free monsterkind, and that he believes that savior is you. He asks you to take his soul and cross the barrier, then he kills himself.
      Asgore: Ha... Ha... I'm sorry I couldn't give you a single, happy ending... But I believe your freedom... is what my son... what ASRIEL would have wanted.
    • Chara/The Fallen Human, if you believe that they genuinely cared about the monster race and wanted to set it free, as opposed to just wanting revenge on humanity.
  • Hey, You!: Almost no one calls the Player Character by name, instead addressing them as "(the) human". This is a hint that the name the player comes up with isn't their name, but that of the Fallen Child. The only times that name is used are by Flowey/Asriel (who treats the player character as a Replacement Goldfish for the Fallen Child) and in the Game Over screen (whose dialogue is actually a memory of the Fallen Child that the player is experiencing). Once Asriel comes to terms with the fact that Frisk is not the Fallen Child, he decides to make Frisk's name known to all the souls he's absorbed before returning them, which causes everyone to start referring to them as "Frisk" rather than "human".
  • Hidden Depths: Every major character has more to them than meets the eye. However, due to the game's extreme level of sensitivity to player choice, it's literally impossible to explore them all in a single run. In particular, going full genocide results in a very strong example of The Apocalypse Brings Out the Best in People, exposing depths that would only be hinted at otherwise. On the flipside, just about everything about Flowey is kept hidden from the player until the end of a true pacifist run.
  • Hidden Track: Only the first 77 tracks out of 101 of the soundtrack are available for preview on Bandcamp. The rest are hidden due to their spoiler-laden nature, and can only be legally listened to by purchasing the soundtrack.
  • Hoist by His Own Petard:
    • Since the Mad Dummy has no interest in ending his fight peacefully, and is immune to physical attacks, the only way to stop him is to trick his little dummies he uses to attack into accidentally shooting him with magic projectiles.
    • Mettaton turns on Alphys when she recruits him to be the "bad guy" so that she can save you from him and he decides it's more fun and exciting to actually be the bad guy who doesn't create openings for her solutions. More notable in that she created his robotic body, which is the primary reason you can't defeat him, since you have no magic and physical attacks can't hurt him.
    • Likewise, Flowey, whom Alphys created from a large golden flower and unknowingly Asriel's dust, traps her and the other boss monsters when they gather to break up your fight with Asgore. Lampshaded when Alphys gets an Oh, Crap! expression right before Flowey captures everyone. However, this almost immediately gets turned on Flowey's head when his transformation back into Asriel Dreemurr causes him to inadvertently regain his compassion, allowing you to calm him down and help him pull a Heel–Face Turn, where he puts down the schemes he made as Flowey for good.
    • Your willingness to do a Genocide Run trips you up when you try to do a subsequent Pacifist Run.
  • Hold the Line: Some bosses and encounters take longer to finish if you're trying to Spare them rather than fight. The player needs to keep hitting Spare (and/or use Act) and withstand waves of attacks until the boss is ready to show mercy (or accept it).
  • Holiday Mode: A minor example: if you've gotten the True Pacifist ending, the Joystick Config menu will be different depending on whether it's spring, summer, autumn, or winter.
  • Hollywood Hacking: Alphys, at several points, hacks into the Hotland and CORE networks to remove certain obstacles for the player. This is, for the most part, a subversion, as she's actually controlling the devices from her computer, but after her plan starts to go Off the Rails in the CORE (thanks to Mettaton), she performs one act of actual hacking to shut down the power there (battling with either Mettaton or the CORE's failsafes in the process). Subverted also in that Alphys is the royal scientist and a gifted technician, so there's really no reason that she wouldn't just have access to these networks to begin with - it's just more impressive if it seems like she's 'hacking in' to save you in the nick of time, because if you were aware that she had access the whole time, there would have been no reason for the obstacle to still be there by the time you arrived.
  • Hopeless Boss Fight:
    • Photoshop Flowey plays with this. When you initially fight him, you can beat him on the first try, but the fight is set up so it seems that you're constantly at a disadvantage: in addition to the real-time battle being disorienting (because of the contrast with the rest of the game) and hectic, your health bar is also massively skewed and each hit you take halves your current health until a certain point, meaning you seem to be down to the last bit for most of the fight. What's more, you initially only do a single HP of damage whenever you use the FIGHT command, and Flowey seems to have thousands. However, you can't die in any of the SOUL segments, and each of them works as a checkpoint that stays completed even if you die. One final twist is when, after seemingly defeating Flowey, he reloads back to full health from the save state he initially saved at the very beginning of the battle, but is finished in a cutscene by the human souls he absorbed.
    • The final boss of the True Pacifist Route seems to present itself as this. You're even told, right before the battle, that it should be technically impossible to defeat him, and that's before he powers up. There is a catch to this. The player can't die. Every time they deplete your HP, your heart refuses and you continue the fight. So it will continue until you win by saving all your monster friends...and, eventually, Asriel himself – you don't defeat him, you use his newfound power to feel love and compassion to remind him of why he's doing any of this in the first place.
    • And the No Mercy ending finishes the set. Except the "boss" is you. Because of your power to save and reset, there's no way the boss can actually defeat you, and they know it. The best they can do is to make the battle frustrating enough that you quit.
    • Downplayed example: While Undyne can be defeated in a Neutral route by attacking her until she dies, she's the only enemy outside No Mercy who cannot be spared by any means. Fortunately for players who are unwilling to kill her, she's also the only boss (other than Toriel) who can be run from...
    • Asgore seems to be this way in a Pacifist run. The 'Hope' in this situation wouldn't be to win, but to spare him and resolve the fight peacefully – and his first move on his first turn is to take his trident and shatter the MERCY button, removing it from the screen entirely. You're then forced to use the FIGHT button, possibly for the first time in the whole game, once AC Ting stops progressing the battle and you run out of items to use.
  • Hope Spot:
    • Asgore inadvertently created one, in declaring war on the humans after his children's death and wanting six souls to cross the barrier, all the while unwilling to do the actual murder. His queen calls him out for this and this ended their relationship.
    • Sparing Asgore on a neutral-pacifist run; sure, you've given up hope of getting out of the underground, but you've ended his genocidal plan amicably and even seem to have found a new family in inclusion to all your friends, then Flowey appears and finishes him off, and things take a nosedive from there.
    • During the True Pacifist ending, Asgore gives a huge smile when Toriel returns to save you from him. She then shuts down his hopes of reconciliation by chewing him out and refusing to be even Just Friends.
    • For Sans, it's hoping during a No Mercy run that you will not kill all the bosses, so that he won't have to fight you.
    • For the Player, doing a Pacifist Run after No Mercy and the world has been ended. Surely you can make things right after going so horribly wrong. Unfortunately, you sold your and Frisk's souls to the Fallen, so that they appear in every subsequent run...
  • Horrible Judge of Character:
    • Several monsters who assumed you were evil and attempted to kill you in a neutral or pacifist run will, in a No Mercy run, when the character is genuinely evil, instead be incredibly trusting of them and simply offer to let them go without a fight, only to be murdered in cold blood. Justified in that if you have been sparing everyone, you will continue to look like a human, but if you have been killing everyone, you slowly start to look monstrous. Humans, as per the narrative, are the enemies of the monsters. There are also a few monsters who are less this and more Genre Savvy, as they're aware that if they try to stand in your way you will kill them, and they hope that by sparing you, you'll stop seeing them as an obstacle and move on.
    • Papyrus plays with this. If you do choose to spare during a genocide run, they will admit that they were a little afraid that you weren't going to accept – and give – mercy, and are glad you did. There's also a little bit of implied fear of He Who Fights Monsters on the boss' end, as he's more willing to spare you and die then kill you in cold blood.
  • Hostile Show Takeover: At the end of your first Neutral run, Flowey ganks Asgore's soul (after killing Asgore himself, if you opted not to), steals the six human souls, and crashes the game. When you restart it, the intro story is interrupted by a glitchy mess, and the save file, instead of your name and location, now reads "Flowey: My World". Eagle-eyed players will also note that the game's title bar now reads "FLOWEYTALE".
  • HP to 1: A number of bosses have attacks that reduce your health to 1, usually as part of Anti-Frustration Features since most of them could easily 1-hit KO a level one locked Pacifist player and having 1 HP gives you at least a chance to heal.
    • It's impossible to be killed by Papyrus, as an attack that normally would will instead bring your HP to 1, and the rest of his attack is halted so he can capture you.
    • Getting hit by any of Asgore's attacks will always leave you with at least 1 HP, unless it's at 1 already. Since he's unwilling to murder, he's holding back until he has no choice but to kill you.
    • The Neutral Route final boss Photoshop Flowey spams attacks and abuses savestates to leave you with 1 HP when he thinks he's won. Running into the circle of bullets while he's doing his speech doesn't hurt you and only annoys him. In fact, Flowey seems to be very fond of leaving you at 1 HP and gloating before sending a killing blow.
    • The True Final Boss pulls this off when trying to destroy the entire timeline. You're only alive because you're that DETERMINED. He will later exaggerate this trope by reducing your health to a minuscule fragment of a HP.
    • If you're on a Genocide Run, the final boss will inflict a poison-like, health draining status and throw attacks at you while you navigate menus, both of which will quickly drain your health to 1, but will never kill you (it would kill you in the beta, but player feedback changed it). Additionally, if you manage to survive his penultimate attack, the boss will repeatedly throw your SOUL against the walls of the bullet board in an unavoidable attack, dealing 1 damage each time, but will also never reduce your HP below 1.
  • Humans Are Bastards: The game builds it up this way at the start – the humans imprisoned the monsters at the start of the backstory because they feared a preemptive strike and decided to attack first. In fact, many people (like Undyne) assume you're a terrible person just because you're a human. This is later subverted: humans are extremely accepting of monsters in the epilogue, and judging by how fast monsters integrated into society, it seems like they want to repent for their previous actions. In fact, when you find out that humans killed a monster who was bringing the body of a human child back to the child's village, the narration states that they believed the monster had killed the child. The human you play as, Frisk, is even an All-Loving Hero by nature. The only human character in the story that qualifies as evil is Chara/The First Child or potentially the Player, and even their case is highly ambiguous: at best they're just a traumatizedsociopathic Well-Intentioned Extremistnote , and at worst they're a Manipulative Bastard-esque Woobie, Destroyer of Worlds who kicked off the entire plot purely for revenge. Either way, if you encounter them at the end of the Genocide Run, it's pretty likely that they've stopped being human entirely — and if the other characters' comments on how you look in their eyes through said run is any indication, so have you.
  • Humans Are Special: Humans, unlike monsters, are mostly physical matter, making a human much stronger than an equivalent monster. Additionally, human SOULs linger after death; this allows monsters to steal their SOULs and use them as a power source, making the monster incredibly powerful. Most importantly of all, human SOULs possess a natural abundance of Determination, which is absolutely essential for your quest to empty out the underground, however you choose to do so.
  • Humans Are Warriors: If you call her outside Gerson's shop, Undyne hints that the war between humans and monsters was a Curb-Stomp Battle thanks to this. It's easy to believe when you remember a single human child is capable of taking down the monster world's greatest warriors by themselves, and that, unlike humans, monsters take (and give) more damage based on the level of killing intent behind the blow.
  • Humans Kill Wantonly: There are many monsters trying to kill you. Maybe they want your soul so Asgore can break the barrier and free the monsters. Maybe they want to prevent Asgore from getting your soul and destroying humanity with it. Maybe they think that you're a horrible monster just because you couldn't afford to spend money on that overpriced bake sale. Or maybe they actually don't want to kill you, but are simply unaware that their actions risk hurting you. You, on the other hand? You kill because you can.
  • Hyperactive Metabolism: Justified, at least somewhat (and Lampshaded), by a particularly curious monster in Grillby's: While human food needs to be entirely digested first, monster food (i.e. anything you consume in the game) immediately converts perfectly into energy upon consumption.
  • Hyperactive Sprite: In battles, most enemies have a simple animation they repeat on loop, with only a few (most noticeably Toriel and Papyrus) that don't animate. It's lampshaded in a Pacifist victory against the Royal Guards, where 01 tells 02 how much he loves "bouncing and waving [their] weapons in sync", which, indeed, is what they do during the battle. It becomes downright surreal with Temmie, whose sprite constantly vibrates with excitement and whose face will eventually vibrate free of her body, and the closer Mettaton gets to attacking with his core for the second time in the main phase of his last boss fight, the faster he starts dancing, to the point of changing poses on every half beat of his theme.
  • Hypocritical Humor:
    • If you go in Sans' room, Papyrus will call him immature... right before asking Frisk if they've seen his action figures.
    • If Mettaton catches you swearing in your essay answer, he delivers this line: "Oh my! This is a family friendly TV show." [ratings drop 150 points] "Now stand still while I murder you."
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    I 
  • I Am Who?:
    • The protagonist has the power to SAVE. This is crucial since it shows that you're not actually playing by the rules of the world. It's acknowledged in-universe by the villain, and the full game really lets loose and discusses how unbelievably overpowered you are.
    • Also, you are YOU — as in, the player. The child you're playing as? That's someone else, named Frisk. You're just influencing them, and they explicitly do things that you didn't command (and refuse to do things that you did command) at certain points in the game. In the pacifist ending, this goes well; Frisk and their friends get a Golden Ending and you leave them be so that they can enjoy it. In the genocide ending, however, this goes horribly wrong; your actions awaken and corrupt the first Fallen Human, who you named at the beginning of the game. They then proceed to possess Frisk and erase the entire game, killing you if you decide not to go along with it.
  • I Ate WHAT?!: Plenty of the food items in the game are made of questionable ingredients. Hot dogs made of water sausages, burgers made of sequins and glue, steak made of anything but meat, junk food made of actual garbage...
  • I Choose to Stay: While it is impossible to choose to remain with Toriel in the Ruins if you wish to progress in the game, in the True Pacifist Ending, you can choose to remain with her. You can also do this by sparing Asgore, giving up your chance to leave the Underground so that he can live. However, this one does not work out, as Flowey kills him, and even if he didn't, Asgore would not accept it anyways.
  • If You're So Evil, Eat This Kitten: In a meta sense, the game does this to the player if they're attempting a genocide run. Notably, you have to murder Papyrus in cold blood and attempt to kill the obviously defenseless monster child.
  • I Gave My Word: In an optional conversation near the end of the Neutral run, Sans says that if he hadn't promised Toriel he'd leave any humans he saw alive, the player would be "dead where [they] stand." It is also the reason why he first fights the player near the very end of the No Mercy run: He had been trying to honor his promise to Toriel for as long as he possibly could, even in spite of the death and horror the player was inflicting upon his friends, acquaintances, and even family, but by the time you meet him before facing Asgore, he has realized that far too great things are at stake to allow a monster like the player to live. He pulls out all the stops to make sure that you don't get past him.
  • I Love the Dead: Played for Laughs during Mettaton's Quiz. One of his questions is "Would you smooch a ghost?" and ALL the answers are "Heck Yeah!" This becomes more significant after you learn that Mettaton is probably Napstablook's cousin (thus, a ghost) in a robot body.
  • I Know You Know I Know: If you've already befriended Papyrus in a previous run and restarted, he wonders if he's met you before.
    Papyrus: DO I KNOW THAT PERSON?
    Sans: do you not know who you know?
    Papyrus: OF COURSE I KNOW WHO I KNOW! I WANTED TO KNOW IF YOU KNOW... I KNOW WHO I KNOW AS MUCH AS I KNOW WHO I KNOW! ... YOU KNOW?
  • "I Know You're in There Somewhere" Fight:
    • Twice. The first is in the neutral ending, during the boss fight against Flowey, as you Act to help the human souls break free of his control. The second is in the true pacifist ending, during the boss fight against Asriel, as you Act to free the "Lost Souls" of your friends and calm down Asriel himself.
    • Happens in the No Mercy run, with Sans pleading for you (as in, you, the player) to remember your humanity. Subverted in that, should you choose to spare him, he traps you and destroys your soul, killing you. Double Subverted since the only remaining way to turn back on your evil ways at that point is to quit and start over. On the Game Over screen, Sans reiterates this by telling you "if we're really friends, you won't come back".
  • I'm a Humanitarian:
    • The Vegetoid enemy is a sentient vegetable. After weakening them with attacks, you can take a bite out of them to recover HP. This was played even straighter in the demo, where you ate the whole enemy — that counted as murder, but the final version doesn't, due to some changes in design.
    • Spider cookies and spider cider, made with real spiders, by spiders, for spiders! Muffet herself pours you a cup full of spiders during the very boss fight with her. Of course, real-life spiders are often cannibalistic, so this is totally normal for them.
  • I'm Cold... So Cold...: You eventually encounter a monster whose check and flavor text is "It's so cold." She's one of the most pitiful fights of the game; her attacks are listless and a few don't even enter the battle box. Unlike the other amalgamates, Snowdrake's mother seems to barely even be alive.
  • I Need to Go Iron My Dog: "I need to go to the bathroom" seems to be a catch-all for monsters excusing themselves but not having anything better to say, since monster food doesn't go farther than the stomach. Papyrus does it to excuse himself, then proceeds to jump out a window and run off, and Alphys does it and runs into another room to avoid having a panic attack (and it also disguises the fact that it's an elevator to the True Labs).
  • Incompetence, Inc.: MTT Resort, as Burgerpants puts it, is a "Labyrinth of bad choices." Right as you enter the lobby, you're greeted by a fountain with the image of Mettaton spitting water right onto the carpet. The residents never get their room service, the janitor is too busy cleaning up his own messes that he can't clean anything else, the elevator's completely busted, and you have to reserve your dishes and silverware to eat at the restaurant. Not to mention that the store sells parfaits and burgers made out of sequins and glue. Burgerpants mentions that Mettaton vetoes any actual improvements they make because "that's not how they do it on the surface."
  • Incredibly Lame Pun: One of the most critical lines of the game's climax, "But it refused," is actually a pun. Frisk refuses to give up during the fight with Asriel; with their Determination as strong as it is, their SOUL, which is split in two on-screen, actually fuses back together.
  • Infinity -1 Sword: The Worn Dagger and the Heart Locket, which belonged to Asriel, are some of the strongest equipment in the game and the final pieces of equipment one can find on a normal or Pacifist run. They don't need to be bought, either, and are difficult to miss, ensuring the player at least has something for the end boss if they've been neglecting equipment thus far.
  • Infinity +1 Sword:
    • On a No Mercy run, the Infinity-1 Swords are replaced by the Real Knife and The Locket, which belonged to Asriel's sibling, the Fallen Child. They increase your stats by 99, but by this point, there is only one enemy left, who's immune to their effects. The one time you really get to see the Real Knife's long-awaited power? When the Fallen Child slashes the world for endless damage!
    • The strongest piece of armor in the game which you actually do get to use is the Temmie Armor, and it is appropriately hard to get. First, you need to discover the secret Temmie Village. Then you need to buy the Temmie shopkeeper a college education (for 1000 gold). Only then will she sell you the armor... for 9999 gold! The price is reduced if you die a lot, but even after 23 deaths, it's still expensive at 750 gold.
  • Improbable Infant Survival: No matter which path you take, Monster Kid always survives his close brush with death. In the pacifist/neutral path, either you or Undyne will save him from falling off of a bridge. In the Genocide path, Undyne saves him from you.
  • Instant-Win Condition: Several fights can be avoided entirely by a single act.
    • Whimsun and Moldsmal don't want to fight at all. All you need to do is spare them, and the fight ends.
    • Muffet will let you go instantly if you eat the Spider Donut or Spider Cider during her fight, as it proves you did participate in her bake sale. Also, if you grind out the coins for it and buy one of the super-expensive items at the bake sale in the Core, Muffet won't even take you into the battle system.
      • Muffet will also spare the player instantly if they're on a Genocide Route, because despite all the murder you've committed, you haven't harmed a single spider. note 
    • Downplayed in the True Pacifist final boss; if you eat the Butterscotch Pie while trying to save Toriel and Asgore, you'll be a single act away from saving them both. In the same battle, saving one of them will instantly save the other even if you didn't act on the latter. The same holds true for Sans and Papyrus.
    • Although not a single act, Mettaton EX ends his final boss fight if his ratings go above 12,000, or 10,000 if he lost his legs.
    • Any dog type enemy, including Endogeny, can be spared immediately with the stick.
    • Endogeny again, using the Hush Puppy. However, through hacking, it has been shown that it does not work on other dogs, despite claims that it neutralizes dog magic.
  • Interface Screw:
    • Several bosses recolor your heart, which changes how you can move it while defending.
    • In an encounter with Pyrope, turning up the heat causes the text box and bullet area to wiggle in the heat.
    • Mettaton adds a "Ratings" meter to the UI; the Ratings increase with theatrics, and getting them to a certain level is how you Spare him.
    • Asgore starts out his battle by directly attacking the Mercy option, so you can't try to spare him.
    • On the Neutral (or imperfect Pacifist) path, Flowey turns the fight into a real-time bullet hell, with the ACT and FIGHT buttons appearing at regular intervals.
    • The final boss of the No Mercy playthrough is capable of attacking you pre-emptively, dodging your attacks, ignoring Mercy Invincibility, and damaging you in between turns by targeting your cursor.
    • On the neutral path, after Flowey kills the king, the game window automatically closes. When reopened, the opening cutscene glitches out and then goes straight to his boss battle, where Flowey even abuses savestates to try and mess with you. Similarly, should you encounter them, the first Fallen Child will give you a short speech, then seemingly attack the game itself, closing it almost instantly. When you reopen the game... you are greeted by nothing at all.
    • The amalgamations in Alphys's true lab all play with this in some way, like having glitched out names, or bizarre and nonsensical actions.
    • Trying to Talk to Madjick will result in him spouting magic gibberish that confuses you, resulting in the SOUL controls being inverted until you either clear your mind (which can also be done before talking to prevent this) or spare him.
  • Inventory Management Puzzle: To a degree. The Child can only carry eight items at a time, not counting their current armor and weapon. This will fill up fast because of stockpiling healing items. To help out, the game provides a "Dimensional Box" in numerous locations that allows you to swap out items whenever you want, which is useful for storing specific foods for later events or for old weapons until they can be sold. The limitation of the box (it's only in a few select locations) becomes moot with Alphys' cellphone upgrade, which gives you permanent access to the box and a second one via the phone (though not during battles); you can't get this in a No Mercy run, but several more Dimensional Boxes appear near the end of the game as a compensation.
  • Ironic Echo:
    • In the normal route final battle, Flowey taunts the player, challenging them to cry for help, for someone to save them, followed with "But nobody came". In the No Mercy Run, he himself describes this as his exact reaction upon first finding himself as a flower.
    • On one of the tapes with Dreemurr home videos, you can hear Asriel asking the First Child to "do their creepy face". In the No Mercy run he, now in his Flowey form, eventually becomes terrified of you and angrily says: "S-s-stop making that creepy face! This isn't funny! You've got a SICK sense of humor!".
    • At the end of the normal route, Asgore introduces himself by saying, "Nice day today, huh? Birds are singing, flowers are blooming. Perfect weather for a game of catch..." At the end of the No Mercy route, Sans echoes the line: "it's a beautiful day outside. birds are singing, flowers are blooming. on days like these, kids like you... Should be burning in hell."
    • During a No Mercy Run, Papyrus will try to change your ways, telling you "EVERYONE CAN BE A GREAT PERSON IF THEY TRY!" Later, Sans says, "do you think even the worst person can change...? that everyone can be a good person, if they just try? heh heh heh heh..."
  • Irony: A game that is heavily critical of replaying it makes it, by far, one of the most replayable games ever. One might ask, "What if I spare this boss?" "What if I spare that boss?" "What if I do a No Mercy run after a Pacifist run?" "What if I do two No Mercy runs?" "What if I do two pacifist runs?" etc, etc. Though given the game's overall message, this could be completely intentional by making you choose, honor Flowey's last request to let the happy ending be? Or reset everything to scratch the itch?
  • I Surrender, Suckers:
    • A particularly cruel way of dispatching Toriel. In her final moments, she lets out a desperate laugh and says you're going to fit in with the monsters outside just fine.
    • Attacking a monster who doesn't want to keep fighting will always do massive amounts of damage; however depending on if you're doing a neutral or a full No Mercy run, it may be more or less effective than just killing the monster outright on the first turn. Papyrus will always spare you at most one turn before you deplete his HP, so there's no way to kill him without invoking this trope.
    • The surprise final boss of the No Mercy run offers you reconciliation. If you do accept, prepare to get dunked on.
  • It Amused Me: During the battle at the end of the genocide route, this is brought up up as a possible motivation for your actions, and it may not be entirely wrong. You are, after all, playing a video game to entertain yourself and be happy, right? The boss then brushes the idea off, though, telling you "but that's ridiculous, right? yeah, you're the type of person who won't EVER be happy."
  • It Gets Easier: This is the true meaning of LOVE, AKA "Level Of Violence". The more you kill, the easier it is to distance yourself and inflict pain, and the less your own soul hurts when fighting, which is represented by increased HP.
  • It Makes Sense in Context: In Hotland, you must use a jetpack cell phone to reach a can of substitute on top of a counter that’s as high as two skyscrapers, or else a flamboyant robot will take your soul to use in a cake.

    J 
  • Jump Scare:
    • In the No Mercy path, during the ending, if the player chooses not to erase the world, the Fallen Child suddenly lunges toward the screen with a Nightmare Face and a horrific noise playing as the previously black background suddenly begins flashing red. The one solace is that your dialogue cursor does not default to "DO NOT".
    • In the Pacifist path credits, the shot of Mt. Ebott is suddenly replaced by Asriel's powered-up form asking you if you thought it was really over. Subverted, as he then reverts to his non-powered "child" form (with a sheepish grin on his face) and he just wanted to tell you that you still need to watch the backer credits.
    • If you do a pacifist run after a genocide run, everything goes great until Toriel walks out of Frisk's room & Frisk opens their eyes & reveals themselves to still be Chara with a nightmarish smile & cackle. Unless you choose not to stay with Toriel. Then, you get the usual photo of everyone together... except Frisk has been replaced by Chara, and all of the other faces have been crossed out.
  • Just Friends:
    • How Papyrus ends your date, because he feels that there is no way for him to match your infatuation for him. The whole scene is played for laughs and is a parody of Dating Sims. The date with Alphys ends this way as well, since she only went out with you because she mistook Undyne's love letter as yours.
    • Subverted when Asgore asks Toriel if they can be this in the Golden Ending. She responds with a blunt "no," though Alphys hopes they will reconcile.
  • "Just Joking" Justification:
    • Royal Guard 01 tries this after he confesses to 02, when it briefly looks like the latter doesn't reciprocate.
    • After Sans implies that he would have killed you the moment you entered Snowdin had he not made a promise to Toriel, he tries to break the ensuing tension by telling you he was just joking (in reality, his threat was meant to show that without his protection, you would have died several times during your journey, such as when he distracts Undyne on the bridge).
    • If you tell Bratty and Catty that you are a human after the completion of the pacifist route, Bratty will try to cover up their previous excitement for the destruction of humanity by saying that they were "just, like, joking, you know?"

    K 
  • Karma Meter: It is revealed near the end of the game that EXP actually stands for "Execution Points", and LOVE stands for "Level Of Violence". These stats essentially act as the game's Meter.
  • Kent Brockman News: Mettaton runs a sketch like this, featuring you.
  • Killed Mid-Sentence: While mocking the player for falling for one of his tricks, Sans gets pretty much sliced in half and fatally wounded. Everything he knows about the game's mechanics doesn't help him; since this is basically cheating the battle system by having the character attack twice during their turn, he does not see it coming.
    Sans: heh, didja really think you would be able-
  • Kill 'Em All:
    • The monsters you meet in random encounters aren't infinite. If you keep killing them, even the monster NPCs will disappear. Doing this for all areas, though incredibly tedious, will give you a vastly different (and darker) ending.
    • If there are leftover monsters remaining, they will flee for their very lives.
  • Killing Intent: The player's during a No Mercy run is so strong that Doggo is able to detect it even though they're standing still. It's actually a plot point, especially in No Mercy. As the player kills more and more monsters, their intent to kill grows stronger, as represented by EXP and LOVE, and monsters are particularly vulnerable to high levels of it. This is why having high LOVE makes the player stronger in combat, and why so many bosses in a No Mercy run fall in a single hit.
  • Kill on Sight: Asgore declared that any humans who appeared in the Underground are to be killed following the death of his son at human hands. He regrets the decree immediately once he calms down, but by that point it's too late to rescind the order.
  • Kleptomaniac Hero Found Underwear: You can search Toriel's clothes drawers at the start of the game, prompting the message "Scandalous! It's Toriel's sock drawer," parodying this trope. Sans's dirty sock pile in his room gets a similar reaction.
  • Klingons Love Shakespeare: Because of all the stuff that flows down from the surface world, monster culture has absorbed a lot from human culture, to the point where it's pretty much impossible to say where one stops and the other begins. Most notably, Toriel has a brand-name chocolate bar in her fridge, and both Alphys and Undyne are very dedicated anime fans.
  • Know When to Fold 'Em:
    • The best way to win the fight with Undyne is to flee from her, at multiple points, when you get the opportunity to flee, and make your way towards Hotland.
      • Undyne herself seems to fall into this if you do the above. After you give her water, she just gets up and walks away instead of chasing you further likely because she would overheat again.
    • In the fight with Asgore, you have to beat him to a near pulp to win because he destroys the Mercy button. Once that happens, he confesses that he did wrong in restarting the war against humans, because it cost him his wife and he never got his son back. Then he tells you to take his soul and cross the barrier.

    L 
  • Land of Faerie: The Underground, which the Monsters were banished to from the rest of the world, filled to the brim with curiosities and oddities alike. All they need is someone to get them out, and that's where you come in. One way or another.
  • Last-Second Chance:
    • The game offers you a meta one during the final cutscene of No Mercy, if you're observant — after the Fallen Child takes control and murders Asgore without your input, they'll turn on Flowey. When he reverts to Asriel's face and voice and begs not to be killed, the Fallen Child falters, and you have to push the button to make them kill their adoptive brother. Alternatively, you can hold ESC, quit the game, and Set Right What Once Went Wrong, but if you got that far without giving up, you probably won't do it now.
    • However, there is one final last chance after this scene. When the Fallen Child gives you the option to either ERASE or NOT the world, choose neither, but instead close the game manually like any window or use the Task-Manager, as the ESC or Q buttons may not work. Most of the time, though, people will fall victim into choosing NOT.
    • Also, this is invoked by Sans when he gives you a last chance to choose mercy during the fight with him... then kills you if you take it. From a certain perspective it's not even a subversion, since he specifically points out that your only option at this point to avoid a bad ending is to quit this playthrough entirely.
  • Last-Second Ending Choice:
    • If you've been sticking to the No Mercy route since the beginning of the game, Mettaton NEO will be your last boss fight where you can choose to ditch your No Mercy run by not killing enough monsters in Hotland. If you decide to kill them, then you're stuck on the No Mercy route for good.
    • At the very end of the Genocide route, the first Fallen Human seems to offer you this. They'll kill you either way.
  • Last-Second Word Swap:
    • At the beginning of the game, if you keep avoiding Flowey's "friendliness pellets", he'll grow frustrated, telling you to "RUN. INTO. THE. BULLETS." – before quickly changing the text to say "friendliness pellets" instead.
    • Undyne does this in one of her phone calls.
      Undyne: If not for that grass, I'd have kicked your...
      Undyne: Uh, if not for that grooty, I'd have kicked your booty!
      Undyne: Don't ask me what a grooty is!!!
  • Lazy Artist:
    • Parodied. Alphys's bed folds up into a "conveniently easy-to-draw cube". Also, Sans's various stands use the same sprite of a snow-covered hut even in the middle of a volcano (which is lampshaded by a nearby NPC); this is Leaning on the Fourth Wall, considering that Sans himself is incredibly lazy.
    • Taken even further when you realize that Sans's sprites themselves never change, they just have details added to them. Even during the fight with him, all that changes are his eyes and his arm. (Until the sweat appears on his forehead.)
    • All of the CORE enemies have a generic out-of-battle sprite that doesn't resemble any of them. Lampshaded in the epilogue when Madjick comments that he looks like a "black sausage" from this angle.
    • There are five video tapes you can "watch" near the end of the game. All of them either take place in total darkness, or the lens cap is still on.
  • Leaning on the Fourth Wall:
    • Less leaning, more like actively pressing into it. The game has a meta-story component to it that is very scathing (yet can also be very optimistic and proud) towards the nature of videogame players that are aloof 100% completionists, karma systems in videogames, and the nature of violence in the medium.
    • In a more straightforward example, if you buy enough hot dogs from Sans, he'll note that you must have some kind of thing for hot animals. He's not judging, though; it helps him stay in business!
    • In a No Mercy Run, Gerson taunts you if you try to ask him to FIGHT. He knows you can't do anything to shopkeepers while they're on the screen, so he'll be happy to babble on to you, if it gives other monsters the chance to escape.
    • When you collect and equip the Real Knife, an item that got its name from a rumor about the 2013 demo, the descriptions for it read "Here we are!" and "About time". The Fallen Child seems pretty thankful for your assistance.
    • Flowey takes a shot at the fifth wall in a No Mercy Run, mocking people who want to see it but can't bear to do it themselves in a way that sounds aimed straight at Let's Play viewers.
      • Mettaton has a similar line during his quiz if you say that Alphys has a crush on you; what he says about her watching you on your journey sounds like he's describing people watching a livestream.
    • In the Nintendo Switch version, a secret room can only be entered by opening cyan and red doors via pressing the Joy-Cons' analog sticks a certain way (cyan and red being the colors most commonly associated with the left and right Joy-Cons respectively). This continues in the battle with Mad Mew Mew, where the SOUL is split into cyan and red halves, both of which are controlled by their respective Joy-Cons.
  • "Leave Your Quest" Test: Toriel wants you to stay with her at home. She genuinely means well by it, though.
  • Leitmotif:
    • The leitmotif that appears in "Once Upon a Time" appears to be the main leitmotif of the game. It appears in "Undertale", "Home", "Hopes and Dreams", "SAVE the World", "Can You Really Call this a Hotel, I didn't Receive a Mint on my Pillow or Anything", "Hotel", "Bring it in, Guys" and "Last Goodbye". It's often believed that this leitmotif is associated with either the first Fallen Child or Frisk.
    • "Memory" appears in "Undertale", "His Theme", and "Finale". It is associated with Asriel Dreemurr.
    • "Your Best Friend" appears in "Your Best Nightmare", "Finale", "Hopes and Dreams", and "SAVE the World". It is associated with Flowey.
    • Another notable one is the Ruins theme. It doubles as the theme for Waterfall, and plays during both of Undyne's battle themes (Spear of Justice and Battle Against a True Hero). It seems to be the theme of monsters, as a whole.
    • Undyne the Undying and Mettaton NEO's boss themes have extremely similar melodies; there are a few potential reasons for this, including it being the standard 'Genocide' boss theme or it being the First Fallen Human's own fight theme.
    • Snowy is reused in chilling and disturbing effect later on when fighting the Amalgamate that is the amalgamation of Snowdrake's mother's corpse and several other monsters.
    • It's implied within the game files that "Determination" is meant to be Asgore's theme alongside "ASGORE" and "Bergentrückung".
    • Finally, a particularly devastating one appears in "Hopes and Dreams", which is shared with "Snowdin Town", "Shop", "Dating Start!" and "Dating Fight!" The motif appears in the most lighthearted, welcoming and, to JRPG fans, most expected parts of the game — in a populated town, in a shop, and while interacting with well-respected NPCs. So when the melody shows up in "Hopes and Dreams", during the final battle in the True Pacifist run, in which the villain has absorbed everyone that contributed to those earlier warm and fuzzy feelings and is using all of those characters' powers to boost his own strength, the emotional impact hits hard as the player realizes both the strength they're up against, and whom they are fighting for.
  • Lethal Joke Item: The Instant Noodles. When you first use them in battle, they take a full minute to prepare, and they only heal 4 HP. However, if you use them out of battle, they heal 15 HP, which is pretty decent for a Pacifist Run where you'll only ever have a max of 20 HP. And if you're in "Serious Mode" (certain boss fights in Pacifist and Neutral runs and all the time in a Genocide run) and you use them in battle, you'll just eat them dry and heal a whopping 90 HP, which can be a lifesaver against the final boss of a Genocide run.
  • Lethal Lava Land: Hotland, the last major region of the Underground, is dry, volcanic, and has many fire-based enemies. It also has some Eternal Engine elements, given that Mettaton has reign over it.
  • Letting the Air Out of the Band:
    • The music in the tutorial peters out if you choose to consistently avoid Flowey's friendliness pellets to his increasing agitation.
    • This happens to "Your Best Nightmare" in Flowey's battle during the SOUL attacks, after you "call for help" via the Act button and before the bullets are replaced with healing items.
    • While roleplaying, Alphys goes overboard and declares her love for Undyne at the top of her lungs while Alphys' theme plays at a much faster tempo and pitch. When Undyne hears her and comes over, the music slows and dies down as Alphys realizes what she did.
  • Level Drain: Downplayed by Napstablook. "Killing" them loses you one experience point. But it's actually a subversion – you lose one experience point, not one EXP (Execution Point).
  • Literal Metaphor: When Undyne says, "There are plenty of fish in the sea" to Asgore, Alphys takes it literally, since she's had a crush on Undyne and Asgore.
    Alphys: S-sometimes you've just got to, uh... S-stop going after furry boss monsters and, uh... J-just get to know a really cute fish...? ... It's a metaphor.
    • Ends up as a Brick Joke since a character at Grillby's makes a similar analogy way earlier, only to explain that he indeed threw out a fishing line and wants to make out with an actual fish. When going back in the playable epilogue, he will even mention that he couldn't catch anything, and that isn't a good method to get girls after all.
  • Literal-Minded: A fishlike NPC in Grillby's tells you he "put out a line" for some girls and is taking the "plenty of fish in the sea" phrase literally. The fishing pole can be found north of the ice room with the save point, and there's a note with a phone number at the end of the line as bait.
  • Look, a Distraction!:
    • In a Pacifist Run, you get Mettaton to turn around using this so you can flip the switch on his backside. Unfortunately, this switch, instead of shutting Mettaton down, actually activates his Mettaton EX form, which starts the real boss fight. Should you die the first time against Mettaton, you can use this right at the beginning of the fight to skip his monologue and the initial Mettaton Battle and skip right to fighting Mettaton EX.
    • This also comes into play in a No Mercy Run, where you try to trick Monster Kid into turning around so you can get right into the fight with him and kill him before Undyne shows up. It doesn't work, and you still have to fight Undyne the Undying.
  • Loophole Abuse: A meta-example: if you quit a No Mercy run by pressing ESC right before the Fallen Child kills Flowey and reset, you avoid the corrupted ending that comes from completing it. Flowey is an Ungrateful Bastard about it the next time you see him.
  • Losing Horns: Sans plays a trombone to mock Papyrus when the Annoying Dog steals from Papyrus's bone collection. Lampshaded by Papyrus.
    Papyrus: SANS, STOP PLAGUING MY LIFE WITH INCIDENTAL MUSIC!
  • Love Letter Lunacy: In a Pacifist run, the letter you give to Alphys on Undyne's behalf is unsigned, and Alphys assumes that since you delivered it, you're the one who wrote it.
  • Low-Level Run: Enforced in the Pacifist route. Since the only way to gain EXP and LOVE is to kill enemies, you'll be stuck at level 1 for the entire game. Which is for the best as "EXP" and "LOVE" stand for "EXecution Points" and "Level Of ViolencE", respectively.
  • Luminescent Blush:
    • Mettaton's quiz show in Hotland is done in the battle system, so the monsters appear in black and white. But when he asks who Alphys' crush is, if you answer Undyne or Asgore, she blushes red in the battle system.
    • This is also present on the Tsunderplane enemy, getting close (but not too close), which translates as hitting a green outline on their attack, causes them blush more and more, eventually making them spare-able.
    • Papyrus also blushes a lot when the player is on a date with him and later when Toriel recognizes him.
  • Luxury Prison Suite: Parodied when Papyrus captures the player a second time. He claims that this time, the prison will be so nice that the player won't want to leave. The prison is the exact same... except for a hot dog cut up in the food bowl.

    M 
  • Made from Real Girl Scouts:
    • Several times throughout the game, there are references to the spider bake sale. It seems innocuous enough. You put money in a web and spiders bring you a spider donut or some spider cider. However, one sign, encountered early on, makes it a little more unsettling.
      Sign: Come eat food made by spiders, for spiders, of spiders.
    • Spider Cider description: "Made with whole spiders, not just the juice."
  • Made of Magic: Monsters are described as having bodies made mostly of magic, as opposed to humans' bodies being mostly water.
  • Magic Countdown: During the bomb defusing segment of the game, you're given 2 minutes to defuse all the bombs. The last 10 seconds are noticeably slower than the first 110. This is an indication that the segment is something of a Worked Shoot.
  • Marathon Boss:
    • Most major bosses are this in a Pacifist run, ACT alone won't make them tame and you'll have to repeatedly spare them and just weather their attacks until they choose to stop attacking you or a third party intervenes.
    • If the player goes on the No Mercy route, most enemies go down in a hit or two, except Undyne the Undying and Sans. The battle with the latter lasts 25 turns, more if the player doesn't attack Sans every turn, since attacking him is the only way to progress.
  • Mass "Oh, Crap!":
    • ALL of the Underground during a Genocide Run, to the point that Alphys takes all the survivors into hiding.
    • The boss monsters when Flowey catches them in the Golden Ending and again, all the monsters of the Underground when Flowey absorbs their SOULs.
  • Maybe Ever After: In the True ending, Toriel and Asgore are shown to have at least reached enough of an understanding to work together at the school Toriel opens. It's not explicitly stated (and unlikely unless Toriel forgave him) that they got back together, but who knows?
  • Meaningful Background Event: Inspecting some areas and background objects reveals a security camera inside. It's Alphys observing you.
  • Meaningful Echo:
    • If Toriel is not killed in her battle to keep the player inside the ruins, she can be found at the very beginning of the game. In the Playable Epilogue, her son, Asriel, is found here instead. Both of them have the exact same thing to say.
      Toriel/Asriel: Don't worry about me. Someone has to take care of these flowers.
    • One of Asriel's last lines in the epilogue, before wishing you farewell.
      Asriel: Don't kill, and don't be killed, alright?
    • When Toriel first meets you, she saves you from Flowey. In the True Pacifist ending, she stops Asgore and you from fighting in a similar fashion.
  • Meaningful Name: Asriel Dreemurr. The obvious conclusion is that Dreemurr is an anagram of 'murderer', but Azrael is the angel of death and renewal in the Bible, fitting quite well with Asriel's full powered persona as "The Absolute God of Hyperdeath" (meaning, perpetual death and instant renewal).
  • Medium Awareness:
    • Everyone in the game seems to display some level of this, all of them without ever quite Breaking the Fourth Wall. They acknowledge that the player is a unique entity, but they always address the Player Character, not the player themselves. With one notable exception: after completing a True Pacifist run, upon re-opening the game Flowey will appear, saying that there is one last threat to everyone's happiness: you, the player. He pleads with the player to not reset and "let Frisk live their life," specifically differentiating between the Player Character and the player.
    • In a more lighthearted fashion, using CHECK on some enemies will net amusing responses from them regarding what was just said, even though checking produces dialogue that only you can hear.
      Checking Napstablook: This monster doesn't seem to have a sense of humor...
      Napstablook: oh, i'm REAL funny.
  • Mercy Invincibility:
    • After you get hit by an enemy's projectile, you'll have a few frames of invincibility. This duration can be increased by specific items.
    • However, the final boss in the No Mercy route does not feature any mercy invincibility, as you obviously deserve none by that point.
  • Mercy Mode:
    • The Temmie armor is specifically this. It's ludicrously expensive (likely more money than you'd naturally earn in the whole game, total) to avoid the player buying it instantly and making everything easy, but every time you get a Game Over, the price drops. It's still possible to get it otherwise, but much easier if you've spent 10 or 12 tries on Undyne and don't seem to be making any progress.
  • Mercy Rewarded:
  • Minimalistic Cover Art: The box of the console versions of Undertale show the game's logo against a black background with the city of New Home below.
  • Missing Secret:
    • During the Playable Epilogue during the Golden Ending, the main menu screen, which had been filling up with the characters you befriend over the course of the game, now has a lineup of all of them. There used to be a noticeable gap between Asgore and Monster Kid, with no way to get anyone else to join the lineup. However, as of the January 2016 patch, the gap is no longer there.
    • In Waterfall, during one of the sections where the Monster Kid accompanies you, there's an Echo Flower on a patch of land that seems like a Disconnected Side Area that requires an out-of-the-way detour, but in reality is just plain unreachable. If you use a cheat tool to get to that flower, it'll just say "Error!" like all of the other unreachable Echo Flowers.
  • Modular Epilogue: The base ending's epilogue changes depending on who is alive.
  • Moment of Weakness: Asgore's declaration of war on humanity. He made it in grief and anger over his childrens' deaths, and by the time he realized what he had done it was too late to change his mind without demoralizing his subjects.
  • Money for Nothing:
    • Money's usefulness is heavily dependent on which route you're going: On a Pacifist run, it's borderline useless since the cheapest healing items heal more or less your entire health bar (especially the Bicicles from Snowdin, which are also double use). On a Neutral run, you might have more use for the high end healing items you can get in Hotland because of your increased health pool, but with high LV comes a lot of defense and battles end sooner so you're less likely to need them to start with. On a No Mercy run, it seems irrelevant since everyone's too terrified of you to do business, and even Temmie seems smart enough to instead try to scam you with useless stuff. At the end of the No Mercy run, however, money is vital, because Sans will quickly tear through you if you don't have food to help out. This may mean spending a lot of money on the expensive foods at Burgerpants' shop, even if you saved some of the better, limited ones like Toriel's pie and the Instant Noodles.
    • From a technical standpoint, it's played completely straight. You can get infinite money halfway through the game, and the method to do so isn't very subtle (though it's pretty tedious). In fact, exploiting it is the only feasible way to get the best armor in the game.
  • Mood Whiplash:
    • The first encounter with Flowey goes from sickeningly cutesy to absolutely terrifying in the span of a few seconds; how soon in those seconds depend on whether or not you decide to eat his bullets friendliness pellets.
    • The ending to the intro stage is also much darker than the silly, tongue-in-cheek atmosphere of the rest of the game up to that point.
    • After Snowdin, a hilarious area full of just as funny characters, the next area you proceed to is Waterfall, a dark cave lit up by false stars, with flowers that echo back conversations of wishes to see the surface. The music is much more subdued and carries an air of tragedy as you learn just how awful and one-sided the mythical war really was, and all the while, Undyne is pursuing you as a hulking juggernaut who only wants to see you dead. This is the first point where the game makes it clear it has a backstory that goes deeper than just "kid falls down a hole and tries to get out".
    • The process of becoming friends with everyone is mostly made of funny, over the top Dating Sim parodies. Then you descend into Alphys's true lab, which is something straight out of a horror game.
    • Amid the horror of the Amalgamates and how they came to be, you can encounter the creepy-looking Endogeny... and it acts like a big, weird gelatinous dog. You appease it by playing fetch and petting it until it plays a modified Dogsong, one of the goofiest, most light-hearted songs in the game, especially compared to the unsettling Woofenstein which plays during the fight, which is essentially just a rhythmic, hollow barking sound that slowly speeds up. A jarring shift of tone considering the rest of the place, but a much-needed one to keep it from being too heavy.
    • Sans gets this treatment a few times, adding to their sudden creepiness. The most notable is in the Pacifist run when you are at a restaurant with him, and he tells you that if it hadn't been for Toriel, you would've been dead on the spot once you left the ruins.
    • If you replay the game on Hard Mode and kill Toriel, it gets stopped after the battle and the Annoying Dog shows up, saying that's where Hard Mode ends. Toriel then casually brushes off death to bake another pie, as if the whole fight never happened.
  • Moral Myopia: To a degree; the ultimate goal of the monsters is to break out of the Underground, and thanks to the power of human SOULs, killing you is the easiest way of accomplishing that. This leads to some moral issues when you consider that using force to stop them from doing this will get you criticized, even if you were in danger of being killed yourself, and you are always accused of doing this out of annoyance rather than self-defense regardless of the player's motivations for doing so. However, the number of monsters that attack you intentionally is rather low, and those that do are shown to be conflicted about it, as seen with Undyne (who tries to look for a better reason to kill you before her fight) and Mettaton (who is concerned Asgore will wipe out humanity if he gets a hold of your SOUL). The game seems to be a test of the player's patience, as Flowey mocks during a Pacifist Run, since he suggests that you might kill out of boredom or frustration after dying multiple times.
  • More Than Mind Control: Implied to be the case with your friends when fighting the final boss of the true pacifist run, Asriel. The Lost Souls refer back to their motivations and negative emotions from earlier in the game; reminding them that Frisk cares for them helps them to break free.
  • Morton's Fork: Finish a Genocide Run, and Chara will show up and offer you a chance to destroy the universe. Regardless of your answer, they kill you and do it anyway, crashing the game.
  • Multiple Endings: Three basic variants, though the phone call at the end of a Neutral run has several sub-variants of its own. None of these have official names; every ending's name is a Fan Nickname.
    • Neutral: Basically the default ending, you get this if you don't qualify for the other two. First, you fight Asgore. Then, you can kill him or spare him — but if you spare him, Flowey kills him himself, and then shatters his SOUL either way so you can't use it to escape the barrier. Then he steals the human SOULs and goes One-Winged Angel to fight you as Photoshop Flowey — unless you've gotten this ending before, in which case he figures, why bother? Either way, you end up outside the barrier, somehow, and much later, you receive a phone call from Sans.
      • Toriel ending, sometimes "Neutral Pacifist": Don't kill Toriel, Papyrus, or Undyne, and don't kill more than 10 Mooks. Toriel takes over as queen, and makes a policy that humans who fall into the underground are to be treated as friends, not enemies. She also disbands the Royal Guard (except for Papyrus). Minor dialogue changes occur if you hung out with Undyne, killed anyone after hanging out with Undyne, killed Mettaton, or hung out with Alphys. If you spared Flowey and hung out with Alphys, he'll clarify here that to get the Golden Ending, you have to "finish the date" by entering the True Lab. If you didn't kill any Mooks and spared Asgore and Flowey, you can just reload the save to accomplish this, but otherwise you'll have to start over.
      • "Exiled Queen": Don't kill Toriel, but do kill Papyrus, Undyne, or more than 10 Mooks. Toriel tries to take over as queen, but is overthrown by monsters who hate humans due to your actions. If you didn't kill Undyne, she spearheads the revolution, otherwise Toriel realizes her policy is unwanted and leaves peacefully. If you killed Papyrus, Sans chews you out for it and tells you never to come back.
      • Undyne ending: Kill Toriel, but not Undyne. Undyne takes over and plans to wage war against humanity. If you didn't kill Papyrus, she assigns him to "The Most Important Royal Position", which consists of standing around and looking cute. If you did kill Papyrus, Sans chews you out for it and tells you that things do not look good for you.
      • Mettaton ending: Kill Toriel and Undyne, but not Mettaton. Mettaton takes over as king and makes the underground into his Egopolis. Sans and (if you didn't kill him) Papyrus are his agents, which Sans thinks is ridiculous but Papyrus thinks is awesome.
      • Papyrus ending: Kill Toriel, Undyne, and Mettaton, but not Papyrus. Papyrus becomes king, by process of elimination. He seems to be doing pretty well by just telling everyone to work hard and making spaghetti for them, but it's implied that Sans is doing the real work.
      • Annoying Dog ending: Kill Toriel, Papyrus, Undyne, and Mettaton, but absolutely nobody else. Somehow, a small white dog becomes the ruler of the Underground. It spends all its time sleeping on the throne and does absolutely nothing important. Somehow, this works out pretty well.
      • Anarchy ending, also known as the "No Mercy" Route: Kill Toriel, Papyrus, Undyne, Mettaton, and at least 1 Mook, but don't go full Kill 'Em All: With the death of everyone important, the Underground descends into despair. Sans laments that this is what happens when people like him take it easy. If you killed more than 20 enemies, he also chews you out for that and tells you to go to hell.
      • Alphys ending: Abort a Kill 'Em All run by not depopulating the Hotlands + CORE areanote : Sans hands the phone to Alphys, who declares she has to do something about your murder rampage, and for what she does (and a lack of better candidates) the monsters elect her as their new leader. She only called to tell you she hates you for what she did, and laments that she should have killed you when she had the chance.
      • "Dirty hacker", or "error handler": This ending can't actually be reached; every possible game state will somehow lead to another one. If you could reach it, Sans would admit that he has no clue how you did it, and suggest telling Toby how you got there — but then admit you're probably only seeing it because you hacked the game.
    • Golden Ending, mostly known as "True Pacifist", though the naming screen Easter Egg indicates it was probably supposed to be something like "mercy run". After getting a Neutral ending and sparing Flowey, he'll give you a hint as to how to get here — kill 0 enemies, and hang out with Papyrus, Undyne, and Alphys. You can't actually hang out with Alphys unless you've gotten a Neutral ending, which ensures that you can't get this ending first, but if you killed 0 enemies and gave Undyne the water, you won't have to reset, just reload. You go through the True Lab and learn of Alphys's failed experiments, and how Flowey came to be. Then, when you face Asgore, Toriel interrupts the fight and tells the two of you that there will be no killing today. All of your friends gather... putting them in place for the final boss to take advantage of them. Flowey appears to steal all of their SOULs. With six human SOULs, and the SOUL of every monster in the Underground making up for the seventh, Flowey can finally take on his true form — Asriel Dreemurr. After an intense fight, you save all of your friends' souls, allow Asriel to make peace with himself and you, break the barrier on the underground, and live a life with all your friends, and every monster on the surface in harmony with humans. You either live with Toriel taking care of you, or remain in contact with her as friends. Arguably the true ending, as it's the only one with a credits sequence.
    • Kill 'Em All ending, commonly called "Genocide" or "No Mercy"note , occasionally called "Max LV" or some variant on "a bad time", and the naming screen Easter Egg indicates it was probably meant to be called something like a "murder run". In each of the four areas (Ruins, Snowdin, Waterfall, Hotland+Core), kill Random Encounters until the Anti-Grinding kicks in, making sure not to miss any Unique Enemy, then kill the boss. The tone of the game changes wildly, and every boss except Undyne goes down in one hit. Save points outside the Ruins just read "X left", and then "Determination" once you've killed them all. The monster history sequence in New Home is replaced with Flowey explaining his personal backstory. He caps it off by saying he appreciates you because like him, you're the kind of person who'd just kill anyone who got in your way, including him. Then he realizes, Oh, Crap!, you're probably going to kill him. Sans's judgement in the Last Corridor is replaced with an incredibly difficult boss fight, and when you finally overcome him, you kill Asgore in one hit and brutally murder Flowey. Then the Fallen Child appears. They explain that your actions convinced them that their Unexplained Recovery was a chance for them to kill the enemy and grow strong, and now that you've killed all enemies, it's time to destroy this world and move on to the next. (This means to move on to the next video game. After all, you're done with this one, right?) You don't get a choice in the matter. After getting this ending the first time, the entire game is replaced with a blank screen. If you wait 10 minutes, the Fallen offers to restore the world, if you give them your SOUL. This lets you play the game again, but permanently alters both the Golden Ending and the Kill 'Em All ending. If you Kill 'Em All again, the Fallen Child is baffled that you'd do so after selling your SOUL to restore the world, since it was doing this once that necessitated that exchange in the first place. In fact, they're so baffled that they restore the world for free and tell you to take a different path next time. If you do take a different path and go for the Golden Ending, the very last scene gets a Stinger reminding you that the Fallen is the one in control now, and implying that they're going to Kill 'Em All without you.
  • Mundane Made Awesome:
    • Should you choose to do it (and you need to do it to complete the True Pacifist run), the date with Papyrus plays out like a minigame within the encounter screen, complete with a fancy HUD. Choosing the right answers will cause Papyrus to be visibly shocked and your "date power" to increase (represented by a bar), all the while dramatic music plays as you date him. Do it correctly, and the "date power" bar overflows, showing just how great you are at dating.
    • If you ask the bird in Waterfall to carry you over a disproportionately small gap, a short but grand song (called "Bird That Carries You Over A Disproportionately Small Gap") will play as it slowly flies you across.
    • Undyne treats her cooking lessons this way, showing you such tricks of the kitchen trade as pummeling vegetables with your fists and aggressively heating the stove to the point where her house catches on fire - just for the sake of proving your FIERCE PASSION. This is all after Papyrus defenestrates himself from the room to "let the two of you get to know each other." Possibly served with some discourse about ice cream women in the human world terrorizing everybody with energy spears.
    • Played for laughs if you phone Papyrus and Undyne within the area where the latter first talked to you. Papyrus eventually asks Undyne if she wants to practice monologues with him, which she doesn't. This trope comes in when you phone them again.
      Papyrus: ATTENTION EVERYONE! I AM GOING TO OPEN THE FRIDGE!
      Undyne: Do you have a monologue for EVERYTHING?
    • The flavour text for the save points tends in this direction:
      Narration: Playfully crinkling through the leaves fills you with determination.
  • Murder Is the Best Solution: Flowey seems to think so, and advocates it. In gameplay, however, unless you're majorly overlevelled for the area you're in, most fights take just as long to kill someone than they do to spare them (about three to five turns, depending on the enemy setup), making killing, even in self defense, unjustified.
  • Musical Spoiler:
    • Napstablook's cousin uses a more uptempo remix of their theme, "Ghost Fight", during their boss battle.
    • Similarly, if you listen very, very carefully during, you can hear notes from "Ghost Fight" in Mettaton's "Metal Crusher" theme, hinting at Mettaton's origins.
    • "His Theme" can be faintly heard in "Finale."
    • "She's Playing Piano", the song that plays while in front of Undyne's house before you date her, is a remix of Alphys' theme, which hints towards their future relationship.
    • Observant players may realize "The Power of NEO", a remix of "Battle Against A True Hero", is also shorter than the latter song, hinting at the fact that Mettaton NEO is faking his strength and that the battle is going to end in a single turn.
    • The song "ASGORE" includes lines from Heartache and Determination, hinting at the connection between Asgore and the identity of the one speaking on the Game Over screen.
    • Asriel's theme plays after you spare Asgore, right before Flowey murders the latter. This hints towards Flowey's true identity.
  • My God, What Have I Done?: Let's say you're trying to weaken a boss so you can spare them. Then suddenly, your attack deals ten times the damage and accidentally kills them. Bonus points if you're not trying to Kill 'Em All.
  • My Rules Are Not Your Rules:
    • Sans can sidestep and attack you between turns, something that no enemy in the game has ever done before, and actually acknowledges being sorely outmatched if playing by the rules, hence bending the rules to deny the player victory. He then tries to stall you out for all eternity by never ending his turn. Your character then breaks the fourth wall and cheats Sans right back, delivering the killing blow by moving the interaction box to the command selection and attacking twice, so they manage to stab Sans.
    • Flowey in his final form knows how to SAVE and abuses this ability to land cheap shots after you dodge. Technically, this is following the same mechanics of the game, of which he has full knowledge, as you do. Where this trope comes into play is, in that you're limited to one savefile. Photoshop Flowey has access to save states, and no less than six slots. The closest thing to that on your side requires outright cheating, by screwing with your save folder and/or backing up several old saves, which obviously cannot be done in normal gameplay.
      • Considering that he controls exactly six human souls at the moment, it might be Justified.
  • Mythology Gag:
    • The track "Fallen Down" was one of Toby Fox's contributions to I Miss You, an album of tracks made with EarthBound's soundfont back in 2011-2012.
    • MeGaLoVania, which was originally the final boss track for The Halloween Hack, makes a reappearance as the theme for the final boss of the Genocide route. The song also appears in Cognitive Dissonancenote  and in Homestuck as something of a leitmotif for Vriska Serket. You can only hear the song by playing the game much like Vriska; killing your way through everything to max out your LV and ignoring what NPCs say.
    • Like Dr. Andonuts from Fox's The Halloween Hack, Photoshop Flowey is an abomination that hijacks the game over screen if he happens to kill you. Both of them then laugh at your face endlessly, though whereas Andonuts required a manual reset, Flowey simply shuts the game down instead.
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    N 
  • Never Trust a Trailer: Lampshaded: "Probably not actual game footage?"
  • New Game+: The game keeps track of the player's activities which affects their experience during later playthroughs. Even if the player resets after reaching the end, certain things persist from their previous runs. Inverted in that only clearing the True Pacifist ending lets you do a "True Reset", which carries over nothing. Unless you have reached a certain ending...
  • New Powers as the Plot Demands: Every time Mettaton seems to have you pinned, either you or Alphys comes up with some new power, like a system to hack or a jetpack upgrade to your phone, that the player has never used or even heard referenced in the whole game prior. This is because Mettaton and Alphys have arranged the whole thing so that Alphys can seem like a hero, giving you just what you need at the moment you need it most.
  • Nice Job Breaking It, Hero!: Right before the final fight in the True Pacifist ending, Flowey tells you it's your fault he can reach his true form since your making friends with everyone empowered them to come help you, allowing Flowey to steal all their souls.
  • Nice Job Fixing It, Villain!: In the demo, Flowey leaves a You Bastard! taunt in your instruction manual if you kill Toriel, only to hint to the player on how they can save Toriel.
    Flowey: YOU DIDN'T EVEN TRY TO SPARE HER, DID YOU?
  • Nintendo Hard: Averted for most of the No Mercy run, since the constant killings have made you ridiculously overpowered, except in two cases: Undyne the Undying, a surprisingly difficult boss who emerges after you've already one-shotted Undyne, and Sans, a surprise Marathon Boss who utterly destroys you over and over again, and who you literally stand no chance of beating without cheating the battle system.
  • No Body Left Behind: Presumably lampshading the common fantasy roleplaying game trope, monsters literally turn to dust upon death. Unlike in those other games, here it becomes an important plot point. And is eventually curiously averted by Sans, who bleeds when struck and dies off-screen.
  • Nobody Poops:
    • Played straight, there are no bathrooms in any house in the game. Monster food is pure energy, and a monster comments on how human food is disgusting because it "passes through your whole body." Possibly implied to soon be averted in the playable epilogue as the same monster mentions about how excited it will be to first try human food and then try a bathroom for the very first time.
    • And Zig-Zagged with the Dog Residue item, which is actually things like "dirty dishes left unwashed by a dog," and "jigsaw puzzle left unfinished by a dog."
  • No Fair Cheating: Downplayed. Certain blatantly obvious edits to your save file will cause the game to metaphorically wink at you, but not have any real consequences. For example, if your name is over the character limit.
  • No-Harm Requirement: Undertale gives players the option to do this on nearly every encounter in the game. It's considerably more difficult, since it turns even a random encounter into a puzzle boss and you don't gain more health since you don't get the necessary EXP from killing enemies. Following through to the end, however, unlocks the game's Golden Ending.
  • Non-Action Guy: Most enemies in the early stages aren't aggressive, and some of them are very easy to SPARE. Given Flowey's claims of "kill or be killed", this would appear to be very atypical of the underground. Or so one might think from the demo alone. Monsters on the outside aren't much less docile than the ones inside. Flowey may have simply been trying to make you think otherwise.
  • Non-Human Head: Several of the NPCs, including the receptionist at the MTT lobby who has a giant hand for a head, and a few nameless NPCs with black and pink-or-yellow diamond-patterned squares for heads.
  • Non-Malicious Monster: Even though they exhibit weird behavior that may harm you, most of the monsters you encounter in battle bear you no ill will.
  • Non-Standard Character Design: Burgerpants wouldn't look out of place in Ren & Stimpy. His face, and the rest of his body for that matter, don't stick to one shape between his different expressions.
  • Non-Standard Game Over: All of the three final bosses have one.
    • Photoshop Flowey begins the typical game over dialogue in Asgore's voice... only to reveal it's really him, laughing about the nightmare you'll never wake up from. Then he closes the game.
    • The final boss of the No Mercy Route has one if you opt to take up his Mercy offer, after which he traps you in a truly undodgeable insta-kill bone prison and mocks you from beyond the grave with a goofy game over theme and an assertion that, after all that you've done, he'll never be your friend again.
      Sans: geeettttttt dunked on!!!
    • This also happens, combined with Fission Mailed, during the Asriel Dreemurr boss fight; your SOUL cracks... and then fuses back together.
  • Noodle Incident: If you call Papyrus and Undyne in the Apron room in Hotland, they'll reminisce about a time they "cooked" popsicles. All they reveal to you is that whatever they did created such a mess that it "took forever to hose everything off".
  • No-Sell: Certain enemies, such as the Mad Dummy and Mettaton, are immune to the physical attacks inflicted with the Fight command. They must be defeated through other means. In addition, Sans will step out of the way of your attacks, refusing to just stand there and take them.
  • No Sidepaths, No Exploration, No Freedom: Looking from certain high vantage points of Snowdin and New Home imply the Underground is actually enormous, but most of your travel is done through relatively small, narrow and entirely linear paths with the occasional tease of other walkways you can't reach (like the conveyor belts where you find the frying pan, or the echo flower across the rainy path you see Monster Kid look at in Waterfall).
  • No Smoking: Typically averted, most prominently with Burgerpants, except in one instance for comic effect: Doggo smokes dog treats in the same way you would smoke a cigarette. You can even find some dog treat "butts" on the ground after you fight him.
  • Not Completely Useless: The Apron is an item that lets you restore 1 HP a turn, and considering the typical length of the battles and amount of damage you receive from bosses, it has no real use for the most part. However, during the fight against Asgore (who will always give you a chance to defend yourself before making a finishing blow), it becomes a true lifesaver.
  • Not in Front of the Kid:
    • Characters will watch their language around you. Undyne replaces "ass" with "booty" in one phone call. On another occasion, Toriel says "hell", then replaces it with "heck".
    • In a True Pacifist Run when everyone in the cast meets at the barrier, Undyne and Alphys obliges Mettaton by kissing, which is interrupted by Toriel saying, "Not in front of the human!"
  • "Not Making This Up" Disclaimer: A sign before a miniboss fight in the second area reads "Warning: Dog Marriage". If you check it again to make sure if you read it correctly, the text confirms that "yes, you read that correctly."
  • Not Now, Kiddo: On the Genocide Route, Flowey realizes you're going to kill him, so he flees to warn Asgore about you. Unfortunately for both, Asgore responds by brushing him aside.
  • Not So Different:
    • Toriel says this to you if you Back Stab her after they lower their guard. In the demo, kill her three times and Flowey mockingly says you're like him. Kill all the monsters and he says it more respectfully.
    • The opening intro leads you to believe that monsters and humans are totally different and the monsters are painted as the evil ones. Should you spare monsters and talk to various NPCs, you can see that they aren't really that much different in humans in terms of what they celebrate, what their favorite hobbies are, and so on, but very few character actually mention the similarities. Should you get the Golden Ending, the trope is further expanded upon where humans and monsters are able to coexist with each other peacefully and very easily since they're very much alike.
  • Not the Fall That Kills You:
    • You survive your fall into the Underground because a bed of golden flowers breaks your fall. Toriel takes care of the flowers for these reasons. The first Fallen Child got injured when they fell, because the flowers hadn't even been planted yet.
    • They also cushion you when the Waterfall current sweeps you away from the second encounter with Undyne.
  • Not the Way It Is Meant to Be Played: The No Mercy run. You will not have a good time, and you will be repeatedly called out for it.
  • Number of the Beast:
    • An extremely well-hidden example: The piece the player is judged just before the final boss, "The Choice", is actually the last part of "Undertale" stretched out to 666% of the original length.
    • This also shows up all over the place when dealing with W.D. Gaster, who appears to be a relative of Sans's. His stats (though also completely pointless, as Gaster has no actual fight) are all sixes, and all of the 'Fun' values needed to see his followers (and possibly get a glimpse of the man himself) begin with 6, culminating in what may or may not be Gaster himself appearing at 66.

    O 
  • Obvious Rule Patch: In the demo, it was possible to gain EXP and increase your LV while still doing a pacifist run by wounding monsters, but then sparing them afterwards. In the full game, you can no longer earn EXP by simply wounding monsters, likely because EXP and LV have far different meanings which would go against the point of a pacifist run.
  • Odd Organ Up Top: One of the receptionists in MTT Resort has a giant hand for a head.
  • Oddball in the Series: Snowdin, not in the series of games, but the series of areas. Snowdin is the only overworld that doesn't use the main overworld musical pattern, the only one where the boss music isn't a remix of the overworld theme, the only one with one large concentration of non-combative NPCs you can talk to, and the one you have to keep returning to over and over again to complete Papyrus' missions for you. It's also the only world where the boss can't kill you even if you try to hit his attacks, as Papyrus (unlike Toriel) will always, always stop when you reach 1 HP and then capture you instead of killing you. If this happens enough times, he'll just offer to let you through. Snowdin is also the location of the only inn in the game, which can restore you to more than full health for free, and the introduction of the dimensional box.
    • Snowdin is likely intended to be a tutorial on sparing. You probably just learned via killing Toriel that beating enemies down until they're tired won't always work, and you may not see the point in sparing them to begin with. How does Toby fix this? Giving you an area where heals are free, so you can take all the time you need to navigate battles and play around with AC Ting, and then throw a bunch of humanizing exposition at you in the form of NPCs in a cute little safe haven town. It's easy to kill the monsters when you see them as enemies, but not as much after you've sat with them in the pub, been doted on by their adults, and talked about how excited they are to one day see the surface and live their lives freely. This is also the only location except the True Lab with Dog enemies, who just want to be petted and played with for a little while and very clearly are just interested in love. To top it all off, any feeble "self-defense" arguments are eradicated since the boss can't kill you and will spare you if you try to kill him. If none of this works to make you feel compassion for the monsters and try to spare them on your own, Sans will outright threaten you about it.
  • Oedipus Complex: It is possible to call Toriel "Mom" and then flirt with her. She will take note of this, and even reference it in the Golden Ending.
  • Official Couple: Undyne and Alphys in the true ending.
  • Offscreen Moment of Awesome:
    • If you do a few multiple Neutral runs in a row while sparing Flowey, he would eventually tell you to stay away from Sans. Because Flowey had tried to complete a Genocide run of his own, but Sans barred his way, and "caused him more than his fair share of resets".
    • Alphys in a Genocide Run manages to evacuate the survivors of the Underground, revealing what she did in the True Lab since that's where they're hiding.
  • Offscreen Teleportation: Sans’s favorite method of travel, which he calls “shortcuts”.
    • Played for Laughs when Sans appears in Gyftrot’s area right outside the mysterious door in Snowdin Forest; when you first walk south into the area, Sans is standing around. As you walk past him towards the door, he reappears at the left edge of the screen just as the right edge of the screen moves past his previous spot. Talking to him will have Sans ask: “say... are you following me?” Moving back and forth in an attempt to have “both” Sanses on the screen at the same time (which is impossible) and then talking to him will have him wonder if you’re lost.
    • If you call Papyrus immediately after finishing the date with Undyne, the latter will join him in Snowdin despite having left her burning house in Waterfall seconds prior. Asked how did she get there so fast, Undyne simply responds: “I ran.”
  • Off the Rails: Mettaton does this twice, first "going off script" during the trivia segment by asking a question about Alphys's many crushes, and then deciding to actually try to kill the protagonist instead of just pretending to.
  • Oh, Crap!:
    • The player when Flowey surrounds you with "friendliness pellets" and prepares to kill you.
    • Flowey when Toriel bats him away in the beginning with a well-placed Fireball.
    • Toriel's reaction when you ask how to exit the ruins twice, followed by her running to destroy the exit.
    • As mentioned under Freeze-Frame Bonus, Toriel sports this expression in the semi-unlikely event that she accidentally kills you in her fight.
    • Asgore's expression when he encounters you for the first time, even taking a few steps back.
    • Also Asgore when Flowey kills him.
    • Flowey again, after his boss battle, when he realizes that the human SOULS have broken free of his control and taken their SAVE files with them.
    • Meta example. Fans of Toby's works would definitely sweat themselves a bit when they hear Megalovania playing during the Final Boss of the Genocide route.
      Game Grumps: Oh! Is that some Megalovania I hear? We are in TROUBLE, boyo!
    • Just before the Final Boss of the Pacifist Route shows up, Alphys pulls this kind of expression upon hearing Papyrus mention a "tiny flower," clearly suspecting that Flowey is the flower she experimented on.
  • "Oh, Crap!" Fakeout: Flowey does this twice:
    • After his Neutral route boss battle, he goes through the stereotypical final boss "explosions and This Cannot Be!" sequence... then he loads file 3. Suddenly, he's back to full health and blasting away.
    • Before the True Pacifist final boss, Flowey seems overwhelmed after the main characters block his attacks, and even moreso after the entire supporting cast piles in to lend their support. Except this was part of his plan too, and he uses the extra monster SOULS to not only regain his true form but become an Absolute God of Hyperdeath on top of that.
  • Omega Ending: An extremely twisted example: If you complete the No Mercy route first by killing everyone in your path and selling your soul to the First Child to restore the world, the True Pacifist ending will be forever tainted by them taking over Frisk and possibly killing everyone. No, True Reset won't undo that.
  • Ominous Fog: There's a heavy fog east of Snowdin that completely blocks visibility. Papyrus fights you the first time you walk through.
  • Ominous Latin Chanting:
    • Wordless chanting appears for a few bars during Asgore's eponymous battle theme.
    • Wordless chanting is heard throughout "But the Earth Refused To Die", immediately before fighting Undyne the Undying.
  • One Degree of Separation: The main NPCs in different areas seem like they're separate from each other, but the end of the True Pacifist route reveals that they're connected much more closely than one might have guessed:
    • Asgore: Undyne's boss and mentor, Alphys's boss, Toriel's ex-husband, Flowey/Asriel's and Chara's father;
    • Mettaton: Napstablook's cousin, Alphys's friend, Papyrus is a fan;
    • Alphys: Asgore's subordinate, Mettaton's friend, Undyne's crush/lover, created Flowey, knows Sans via Gaster?;
    • Undyne: Asgore's subordinate and protegée, Alphys's crush/lover, Papyrus's best friend, also knows Sans;
    • Papyrus: Sans's brother, Undyne's best friend, unknowingly communicating with Flowey;
    • Sans: Papyrus's brother, knows Undyne, Toriel's friend, knows Alphys via Gaster?;
    • Toriel: Asgore's ex-wife, Flowey/Asriel's and Chara's mother, Sans's friend;
    • Flowey/Asriel: Son of Asgore and Toriel, adopted sibling of the Fallen Child (as Asriel); created by Alphys, secretly manipulating Papyrus, has apparently fought Sans before (as Flowey).
  • One-Hit Kill: This is Justified in a No Mercy run; it becomes very difficult for monsters to fight when their opponent has significant hatred, causing most bosses to have their defense stats decreased when you face them.
  • One I Prepared Earlier: Briefly discussed at the end of the "Cooking With a Killer Robot" segment.
    Mettaton: Haven't you ever seen a cooking show before? I already baked the cake ahead of time!
  • Our Ghosts Are Different: Ghosts in Undertale are just another type of monster, although they have all of the typical abilities of ghosts in fiction. Souls are something else entirely (see below).
  • Our Monsters Are Different: "Monsters" are beings made of magic. They start at Funny Animals and end somewhere short of Lovecraft Lite abominations, with the occasional thing you can't really classify (like a tsundere airplane) in between. However, they're all more or less friendly if you get to know them.
  • Our Monsters Are Weird: The principle characters are all fairly "normal"—skeletons, a creature from the black lagoon, goat-people...the enemy monsters and background characters, however, include disembodied heads, beings of pure flame, tsundere planes, people with flashing polygons for faces...
  • Our Souls Are Different: Much is made of the power and properties of souls, both human and monster alike. For one thing, human souls are powerful enough that they linger after death, unlike monster souls which vanish instantly apart from those of "boss monsters" like Toriel and Asgore. This is because humans possess the power of Determination.
  • Outside-the-Box Tactic: It's possible to completely avoid having to fight Muffet. The main way would be to buy an item from her Spider Bake Sale when you encounter it in Hotland, except the treats cost literally as much money as you can possibly carry. However the Ruins has a branch of the sale that sells treats much cheaper. If you buy a spider treat then eat it during the fight with her, she'll let you go right away.
  • Overly Long Gag:
    • A pack of instant noodles almost takes as long to cook in-game as it does in real life. And is about as nutritious. If you're in "Serious Mode" (i.e. in certain boss battles or on a Genocide Run), you eat it raw and it heals up to 90 HP instantly.
    • Petting the Lesser Dog, and watching it stretch itself to impossible lengths. This can go on for a very long time.
    • Ringing the bell on Doggo's station. The poor guy can be tormented by it for good amount of unique responses.
    • If you buy a Hot Dog...? from Sans with a full inventory, he'll put it on your head. Talk to him again, and he'll put another there for free. And then another, and another, until he decides that 29 is enough because he can't reach any higher. If you walk around, they'll come crashing down. Then you can get more by speaking to him.
  • Overly Long Title: Some of the soundtrack tracks, including "Bird That Carries You Over A Disproportionately Small Gap", "Can You Really Call This A Hotel, I Didn't Receive A Mint On My Pillow Or Anything" and "Song That Might Play When You Fight Sans".
  • Overly Narrow Superlative: During a phone call with Papyrus while in Hotland, he states that Mettaton is his "favorite sexy rectangle".

    P 
  • "Pachelbel's Canon" Progression: Appears partway into "Fallen Down (Reprise)", specifically at the part that quotes the game's main theme.
  • Pacifist Run: Not only possible, but the only way to get the Golden Ending is to not kill anyone during the game.
  • Painting the Medium:
    • Sans and Papyrus talk in... well, Comic Sans and Papyrus. The third Skeleton Brother (or perhaps their father, or perhaps someone of no relation at all), W.D. Gaster, speaks in Wingdings.
    • When the Fallen Child destroys the world at the end of a No Mercy run, the game window repeatedly shakes, mirroring the animation for One Hit Killing a boss.
    • During the shows that Metatton forces the player through, the name of the game on the top right of the window changes if you aren't playing in full screen.
    • When you reach the end of the neutral route, Flowey crashes the game to desktop. The opening that normally plays will be slightly different upon booting up again, until it glitches out and launches back into the story immediately. If you die during the final boss, the game will also automatically crash to desktop.
    • Unfortunately due to technical restrictions, many of the above examples don't happen on console versions of the game, due to not having as much free reign with what the software itself can do, such as closing out the game without the player's input.
  • Parental Abandonment: Asriel and through adoption, the Fallen Human, are the only major characters in the entire game who have any parents to speak of. Heck, depending on how you interpret Gaster's relationship with Sans and Papyrus, they're the only named characters outside of a handful of NPCs whose parentage is even hinted at.
  • Peer Pressure Makes You Evil: The Migosp attacks only while it has allies present, but acts like itself (a carefree dancer) once they're gone.
  • Permanently Missable Content:
    • Due to the way save games and the New Game+ system works, certain story elements can be forever lost to the player. Also, getting the bad ending forever taints any future playthroughs, especially the Pacifist ending (barring the player editing the games files directly).
    • Photoshop Flowey can only be fought once on any given save file prior to a True Reset. Flowey still appears after Asgore's death to finish off his SOUL, but decides against absorbing the human SOULs and taking over the game because he remembers what happened last time and knows you'll just defeat him again if he does.
    • After the battle with Toriel, you're locked out of the Ruins until the Playable Epilogue. The primary consequence of this as far as the rest of the game is concerned is the Ruins is the only place you can buy spider pastries cheap, which can make the encounter against Muffet significantly easier.
  • Pet Monstrosity: The Amalgamates to Alphys, particularly Endogeny.
  • Phlebotinum Overdose: One of Alphys' experiments in barrier-breaching research was to inject monsters with human Determination as a substitute for SOULs. This ended up being a very bad idea: apparently neat things happen if a monster gets the Determination threshold just right (such as Undyne the Undying and possibly Sans), but even just a tad too much will cause a monster's magical body to melt. You see this very thing happen to Undyne if you kill her, whose will not to die creates more Determination than her body can handle. The Amalgamates in the True Lab also came about this way when several melted monster bodies fused together.
  • Pistol-Whipping: Since the Empty Gun has no bullets, it can be inferred that this is how the weapon is wielded when you use it.
  • Planet of Steves: Temmie Village. Apparently, all Temmie have "Temmie" as a name. Except for Bob, that is.
  • Platform Hell: Most encounters tend to play as Bullet Hell, but some require extensive platforming, especially the final boss of the No Mercy run. You're gonna have a bad time indeed.
  • Playable Epilogue: In the Golden Ending, you can go back and talk to the NPCs around the world. Like in EarthBound, they will have unique dialogue to share; most of it isn't too important, but back-tracking to the very first room in the game gives you a bit of a surprise.
  • Player Death Is Dramatic: During battle, the human's SOUL is visible when fighting monsters. If the human's HP hits 0, then all monsters on screen, along with the entire interface, vanish. Only the human's SOUL remains visible before cracking, and promptly shattering into pieces, signifying the human has died.
  • Player Nudge:
    • If you fail the Behind the Black puzzle in the Ruins enough, the game will pop a text box telling you that there's a switch behind the first pillar.
    • If you kill Toriel, Flowey will snidely remark that "it's not as if you can go back and change fate". If you're playing the demo, his comment on the last page of the manual will also change, taunting that "you didn't even try to SPARE her".
      • If you've killed Toriel and reloaded a SAVE, then try to talk to her twice, the game will give you a hint on how to get past her without killing hernote . You don't get the same hint if you're facing her for the first time.
    • Completing a Neutral path without killing anyone will have Flowey suggest that it might have been better not to care about anyone at all, hinting at the No Mercy path. Even if you did kill some monsters, as long as you at least spared Flowey, he'll pretty much spell out the exact conditions for a True Pacifist ending.
    • A very subtle one in Waterfall: the area is long enough and has a low enough monster population that if you play the game like a normal RPG and kill every monster you encounter without deliberately grinding, you're likely to fully depopulate the area and get the "but nobody came" encounter. A curious player might then wonder what happens if you reset and do this everywhere...
    • Completing a No Mercy run after you've already done one and sold your SOUL will have the Fallen Child suggest you take a different path, hinting at the True Pacifist path and the corrupted ending you get for completing it without a SOUL.
    • After the 1.01 update, the solution to the Waterfall piano puzzle is displayed on the wall after giving the statue of Asriel an umbrella.
  • Player Tic: People doing Let's Play of this game tend to read everyone's speech bubbles, with or without assigning different voices/inflections to everyone.
  • Playing a Tree: During the ending credits, Burgerpants finally gets his chance to be on stage with Mettaton. Unfortunately, it's as a bush.
  • Playing the Player: In more ways than one.
    • Most players who have played an RPG will know what to expect from the EXP and LV stats: you want to bump each of them to make your character stronger. Except they don't stand for 'experience' and 'level', but 'execution points' and 'level of violence', and your increasing strength represents you being desensitized to the carnage you're inflicting. Having your EXP at 0 and your level at 1 the entire game is a good thing, as you cling to your humanity in the face of everything.
    • Most players are accustomed to games that allow you to erase your progress and start from scratch. It will gradually become clear, as you play, that you aren't the only one in the game who recognizes that this world can be stopped and restarted at a moment's notice, and especially along the genocidal path it has a few choice things to say to those who believe they can massacre everyone in cold blood and then just reboot the game and pretend like it never happened.
    • Finally, there are certain behaviors players are most likely to expect from RPG boss battles. The ability to save, reload, forcefully close the game and erase your save files are not among those behaviors. Once Flowey gets his hands on the human souls, he gleefully indulges in all of them. There's also No Mercy Sans who starts by suckerpunching you with an Alpha Strike, interrupting his Battle Intro to suckerpunch you, dodging your attacks in a game with no evasion stat, attacking you in the menu, and finally doing absolutely nothing so you never get your turn.
  • Point of No Return:
    • Near the start, after you leave the Ruins if you spared Toriel, the door locks behind you, preventing you from going back. There also appears to be one at the end of the game, with a Save Point appropriately named "The End" located right before the final boss, who's waiting in the door right next to it. But even so, along with a request from the final boss that you finish any business now before you're ready to fight them, you can still leave and go back to the rest of the underground after saying you're ready by dying in the fight, the only consequence being that you skip their opening monologue. The actual point where you can't go back to the rest of the underground happens elsewhere, depending on your ending path.
    • On Neutral: Right after you defeat Asgore. Thanks to Flowey stealing the human souls, he locks you out of your save file until you beat his powered-up form and reach the ending. But afterwards, you can load your game to right before fighting Asgore, allowing you to explore again as well as try for the pacifist ending if you haven't killed anyone.
    • On Pacifist: By heading back and entering the True Labs. A power-failure causes the entrance elevator to crash and prevents you from leaving until you restore power and finish the area. But then, when taking the exit elevator, another accident happens and carries you too far, depositing you at New Home. But now, a mysterious tangle of vines has sealed the lift doors shut, preventing you from going back until you reach the Playable Epilogue.
    • On No Mercy: Entering the Throne Room and meeting Asgore, (though the preceding fight with Sans unique to this path can also be this from a thematic standpoint.) The ending then plays out immediately, with no effort needed on your own part, and the game ends. Also, due to what happens in the ending, you can't reload and explore anywhere anymore; not without resetting the game, at least. However, the events from this ending will follow you through the reset and affect your file in certain ways, making it a pretty big point of no return.
    • The No Mercy run itself is this; specifically, the fight with Mettaton NEO. If you don't kill enough monsters in Hotland, he tells you you can't be all bad and lets you go, putting you on the track for a neutral run. Apart from just giving up and quitting, this is your last chance to end the No Mercy run, story-wise.
    • The biggest point of no return happens when you make your deal with the Fallen after the battle with Sans; from then on out, the True Pacifist Route's ending and future No Mercy Route endings will be permanently altered.
  • Poke the Poodle: Among the 'evil' things you can do is take multiple pieces of candy from a bowl that says "take one". Additionally, the game itself does this on the hidden Hard Mode:
    Narration: In this hellish world you can only take 3 pieces of candy.
  • Post-Final Boss: In the No Mercy route, Sans the skeleton is the effective Final Boss, as the next two bosses, Asgore and Flowey/Asriel, are killed via cutscene.
  • The Power of Hate: Monsters are chiefly composed of magic, and are vulnerable to strong, negative emotions. If you play the game violently, you can overwhelm some bosses and kill them almost instantly. In one instance, the Mad Dummy will bond with its body due to the intense hatred it has for you (which opens it to attacks as it is now corporeal).
  • The Power of Love:
    • Frisk is implied to be powered by a mix of love and determination during the battle with Asriel, allowing them to reduce damage from attacks, create healing items and resurrect themselves shortly before their deaths.
    • On the Nintendo Switch, you can encounter Mad Mew Mew, a Mew Mew doll inhabited by the ghost from the Mad Dummy. If you talk to her, she starts to fuse with her new body through love.
  • Pre Ass Kicking One Liner: "it's a beautiful day outside. birds are singing, flowers are blooming... on days like these, kids like you.... Should be burning in hell."
  • Precision F-Strike:
    • Bug Flowey one too many times after getting the neutral ending:
      Flowey: Quit looking at me with that stupid expression. You're pissing me off.
    • Out of all characters, Toriel does this in the Golden Ending. She immediately thinks of the children and backtracks to censor herself.
  • Preexisting Encounters:
    • Most non-plot important enemies in the game are scripted to be encountered in specific areas, though natural random encounters exist alongside them. This guarantees you'll encounter most enemy types at least once because how you deal with them determines what kind of neutral ending you get or what the credits in the Golden Ending say.
    • One puzzle in the Ruins features a couple of Vegetoids half-buried in the ground that emerge to attack when examined.
  • Production Throwback:
    • A remix of MeGaLoVania (a song written by Toby Fox for his EarthBound Halloween Hack which later appeared in Homestuck) plays during the fight with Sans at the end of the No Mercy route.
    • One of this game's Final Bosses, Photoshop Flowey, has a similar game over sequence to the hack's Dr. Andonuts: laughing endlessly and forcing a reset.
    • The last set of minibosses in The Halloween Hack and this game have something very important in common: They're both called Amalgamates.
  • Product Placement: In-universe example during Mettaton EX's boss fight. One thing you do to increase ratings is eat MTT brand foods.
  • Prophecy Twist: The prophecy of the Delta Rune foretells that an angel who has seen the surface will return, and the underground will go empty. Frisk fits the bill... but so do Asriel and Chara. In a Double Subversion, the prophecy comes true in both of the more extreme endings: either Asriel destroys the barrier in the Pacifist ending, or Chara makes the Underground go empty in the Genocide ending.
  • Protagonist-Centered Morality:
    • Subverted big time. Spare the major encounters but kill common monsters on the way? You'll get called out on it.
    • Lampshaded by Alphys: "Watching someone on a screen really makes you root for them".
  • Punny Name:
    • (Tu)Toriel
    • Alphys = Alpha / Physical
    • Undyne = Undying. Also, to a lesser extent, Undines.
    • Papyrus and Sans are a 'font family'. The Retgoned W.D. Gaster is also in on this.
    • Snowdin = snowed in. The inn there is called Snowed Inn. It's also a pun on Snowdon Mountain, a region in Wales.
  • Punctuated! For! Emphasis!: After Undyne recognizes you as the same human that she tried to kill and ended up being saved by, after Papyrus introduces you as a friend.
  • Purpose-Driven Immortality: If you die, your determination keeps you going (and restarts your adventure at the previous save point. If you die to Asriel, your determination tells death to get dunked on.
    Narration: But it refused.
  • Puzzle Boss: On a pacifist/low kill run, all encounters essentially become this, with the 'puzzle' being working out how to end the encounter nonlethally.
  • Pyrrhic Victory: How the player will feel inevitably after getting the Genocide ending: you successfully beat Sans, the hardest boss of the game, and slaughtered everyone. The Fallen Child reveals themself and destroys the very world of Undertale, leaving nothing behind but a black screen to stare at. It is possible for the player to recreate the world at the cost of Frisk's Soul, something which will permanently alter the True Pacifist ending. No matter how you look at it, you, the player, lost: either you leave the game for good, knowing that you are directly responsible for this universe's destruction, or you recreate it, only for the Fallen Child to hijack Frisk's body at the very end and (possibly) kill everyone and everything you hold dear. Your choices no longer matter: whatever you choose, once you reach that point, there is no happy ending possible for anyone, including you, unless you invoke Eviler Than Thou against the Fallen Child.
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