As a story-based game, Undertale foreshadows a lot of elements. Beware of spoilers!
- The demo's manual page about enemy turns has been ruined by "artless hooligans", according to Flowey. Turns out it was probably him, so he could trick you when you first meet him in-game. He can still change the manual after you finish the demo.
- If you put in Flowey's name when you're naming the fallen human, he'll say "I already CHOSE that name," hinting that he was able to choose his own name rather than be born with it.
- Name the "fallen human" itself is this as you are not naming the human child that you're playing as (Frisk) but the original fallen human (Chara), there is a reason when you chose Chara as the name the game itself tells you it's the true name.
- The very intro of the game, the first thing one sees upon launching the game, has subtleties that show that the child that falls in the intro is not the player character: the stripe pattern on Chara's shirt is visibly different from Frisk's and Chara falls onto bare ground whereas Frisk falls onto golden flowers. The intro also features a silhouette of Asgore fighting. Anyone familiar with goat biology who happens to remember this image might figure out that Toriel is the queen far before other players.
- Near the end of the Ruins, the player can find and equip a toy knife. Playing violently up to that point will also cause the narration to ask where the knives are in Toriel's kitchen. And with enough effort, you can indeed find one.
- If you talk to the Snowdin shopkeeper about the Ruins, she says, "unless you're a ghost or can burrow underground, it's impossible to get in". A miniboss fought there is a ghost. Sure enough, said ghost is the only monster besides Toriel that you can meet outside of the Ruins, or, more specifically, Waterfall. In addition, Flowey is also shown being able to burrow underground...
- The entire scene with the training dummy at the start carefully picks its descriptions to personify an inanimate object. Because it's actually the cousin of the Mad Dummy you fight later on.
- The last weapon you can get in the game is the Worn Dagger. Its description reads "good for cutting weeds"... you know, like Flowey.
- Napstablook doesn't have a "damaged" sprite or sound effect. Your attacks aren't actually doing anything, and they're just reducing their HP out of politeness. After "depleting" their health, they leave, and because the battle accomplished nothing, the game tells you, word-for-word, that "You lost one experience point." Observant players will notice that the game specifically says "experience point" here, while normal battles call it EXP; extremely observant players will check their status menu and notice that they didn't actually lose any EXP. This is because "Experience Points" don't exist; EXP, or "Execution Points", is just a term coined by monsters for how someone's ability to kill develops. You don't learn this until Sans's judgement at the other end of the game.
- In Toriel's home, there is a calendar with a date in the year 201X circled. This is the date that Chara fell.
- Said calendar is described as old, hinting at the later reveal that 201X is not the year the game takes place.
- Both Mettaton and Alphys drop hints that Mettaton being a malicious killer is actually a lie to get you to like her. This includes Mettaton drawing out and repeating lines at Alphys' cue which she frequently misses, Mettaton's robotic cough and stopping the 'firewall' before it hits you, him lengthening the bomb defusal time, one of his news ticker headlines lampshading the malfunctions and puzzle reactivation in Hotlandnote , frequently praising Alphys, and picking up her lines when she forgets them.
- Mettaton's true identity can be guessed with many foreshadowings:
- In Alphys' Lab, you can see a letter signed by Mettaton who thanks her to have made his dreams come true. When you talk to Bratty and Catty, they tell you something strange about him: he acts like it was HIS idea to be built.
- At the end of his EX battle, Mettaton opens a telephone line, and the first caller speaks in lowercase letters. Mettaton seems to know them well, even crying their surname in despair. It was actually Napstablook, giving him a last goodbye.
- When Napstablook invites you to their house, you can see they have a TV and they like to watch a show on it.
- Also, you can see another house next to theirs which looks identical, except it's in pink. It turns out to be Mettaton's house, who left the family farm to become a TV star.
- At certain points in the game, if you quickly return to the start of the room you're in, you'll catch sight of Flowey quickly receding into the ground. He's been following you this entire time, foreshadowing his sudden appearance at the Barrier. You led him safely right to Asgore.
- Loads towards the identity of the king and queen. Toriel wears the Delta Rune on her dress, foreshadowing her place as royalty. A Froggit in the Ruins comments that everyone in the Ruins is "too intimidated" to talk to Toriel, hinting early on that she's more than just an ordinary citizen of the Underground. Asgore uses attacks identical to Toriel's, just more powerful. The royal castle is a one-to-one copy of Toriel's home. An incredibly subtle hint is the description of Toriel's bed: "Definitely bigger than a twin-sized bed.", which would make this around a queen size. Returning in the walkaround section after the Pacifist ending (after the relevant revelation) will change the text to "Earlier you identified this as bigger than a twin bed. You realize now it's one more size above a double." In other words, a queen.
- Flowey's theme is titled "Your Best Friend," which fits his Faux Affably Evil nature. This foreshadows his true identity as Asriel Dreemurr, prince of monsterkind, and best friend of the Fallen Child, with whom the player likely happens to share a name. On a Genocide run, the Fallen Child possesses Frisk because of the player's influence, making the title "your best friend" even more literal.
- One of the Froggits in the Ruins warns you that someday you might have to spare someone even if their name isn't yellow (an indicator that you can spare a monster). This is the key to getting through the fight with Toriel without killing her.
- During the No Mercy run, when you first encounter Sans and don't laugh at his joke, he mutters to himself, "gee, lady, you really know how to pick em, huh..." At first it seems like he's talking to you, but really, he's talking about Toriel, who asked him to spare any humans he encountered. Due to the nature of the run, however, you wouldn't discover this unless you had previously done a Pacifist run (though it's heavily implied in an optional conversation with Sans at the MTT resort, in any run where Papyrus was spared).
- When you're on the date with Papyrus, you find a joke book his and Sans's home. Inside the book is a quantum physics book, and inside that is another joke book, and inside that is another quantum physics book, at which point you decide to stop. Though it doesn't make sense out of context, this proves that Sans uses the quantum physics book to grasp the concepts that allow him to use his time machine, and the multiple layers of books within books shows that he's able to send objects back in time, just like he did with the photo of the playable character and the NPCs after the post-pacifist true reset, as he was able to retrieve multiple copies of the same books. Then again, Word of God states that he was never able to fix the broken time machine (and never will be able to), so this symbolism may not quite work.
- In Papyrus's culinary art museum, a.k.a. his refrigerator, you'll find lots and lots of containers labeled "spaghetti" and a single, empty bag of chips. The only other place you can find chips is in the True Lab. This foreshadows Sans's position as a scientist and, by extension, his connection to Dr. W.D. Gaster.
- When Sans warns you that you will have a bad time and disappears during the No Mercy run, it is a strong implication that Sans is self-aware and can access the same meta-concepts as the player, as no other character has been able to simply teleport out of nowhere and reappear at will. He can also do this during the Pacifist run, when you are trying to solve a puzzle at the bottom of a cliff and Sans is at the bottom, waiting at both ends of the screen although we never see him move. He lampshades this by asking if you are following him, and stares straight at the player instead of at Frisk while he's idle. He even does this after telling his awful puns to Papyrus, both times pausing to grin straight at the person playing rather than Papyrus or Frisk when the rimshot plays.
- The second time you meet him in the Ruins, Flowey refers to himself as "the prince of this world's future". His true identity, Asriel Dreemurr, is the prince of monsterkind.
- When Sans is talking about his brother's "special attack", the term itself is highlighted in blue. Actual blue, not the cyan color which Sans uses to talk about "stop signs", so by that logic this "special attack" is something you've never seen before. Cut to the actual boss fight with Papyrus, he reveals that he can subject the SOUL to gravity… by turning it blue.
- An odd retroactive example: during the battle with Asriel, when you try to SAVE Papyrus and Sans, if you SAVE Papyrus first, he'll fall silent, but the attacks will keep coming, hinting that Sans is more powerful than he first appears.
- At one point in the game, you can get an optional conversation with Sans that reveals Papyrus receives flattery, advice, encouragement, and predictions from a talking flower — Sans thinks somebody's playing a prank, but the player should be familiar with Flowey by now. This foreshadows the method Flowey used to trick everyone into coming together during the Pacifist route.
- When you first meet them, Monster Kid remarks that you must be a child since you're wearing a striped shirt. Turns out this character design convention is true for the First Child and Asriel.
- If you end Muffet's fight without killing her, she'll come to the conclusion that her beef with you is a big misunderstanding, and that she thought you hated spiders. She decides that the person who wanted her to steal your SOUL must have been referring to a different human in a striped shirt. Flowey is a dirty liar, and this is just one of many hints that he's been manipulating people against you behind your back.
- The Monster Kidz Word Search, a simple gag in Snowdin, has a few subtle hints towards future events. Not only does it have the four seasons in it (regions in the Underground are themed around them), but it also contains "skeletons", "mermaid", and "robot"; while you already know about the skeletons at this point (they're the ones who set up the word search, after all), this is far before you encounter Undyne (who isn't a mermaid, but is still a fish-based monster) and Mettaton (the only robot in the game).
- During the hangout with Undyne, Undyne tells you when she was a hot-headed kid, she challenged Asgore to a duel. Asgore never fought back and instead dodged all of her attacks. If you haven't met Asgore in a previous playthrough, this is probably one of the earliest dead ringers that Asgore isn't as bad as Toriel paints him to be. While he did kill humans who fell into the Underground, he's become so regretful of doing so that he'll destroy your Mercy button while giving you a fighting chance against him in his battle.
- While poking around Alphys' lab, you can find a half-filled bag of dog food, and a stack of unopened letters from monsters like Froggit, Snowy, and Doggo. Nothing suspicious since there's dogs everywhere in this game and Alphys is an anti-social shut-in, right? If you explore the True Lab, these innocuous items become a lot less innocent — Alphys uses dog food to feed the various Amalgamates sealed down there, and is so guilt-ridden about accidentally creating them that she can't bring herself to talk with the horrors' families.
- If you call Papyrus outside the lab (before befriending Undyne), Sans will suggest that finding dogs in there wouldn't be unexpected, possibly hinting at the nature of one of the amalgamates.
- In the MTT Resort, the fountain's plaque says it was built in 201X and the Mettaton statue was added last week (that is, quite some time after 201X). This foreshadows the fact that 201X was decades (or possibly centuries) ago, rather than being the year the game takes place in.
- If you speak with the various people inside the MTT Resort's restaurant, you learn that the Core can be configured in multiple ways, and that a new configuration has just been finished. Alphys may have intended the Core to be configured specifically for the player, but the fact that the configuration doesn't match her map suggests that someone has messed with it. This turns out to be Mettaton's doing and also serves as a proof that Alphys may not be as savvy as she suggested in Hotland.
- In the room where you first directly encounter Undyne, you can backtrack and hear Toriel worried about your fate (if you spare her) or struggling in nothingness (if you kill her). It turns out that Flowey was taunting you, revealing he has a certain talent for imitations. After the "date" with Alphys, you receive a call from Papyrus, who greets you with a "Howdy", speaks in a suspicious way, and suggests that you should go in Alphys' lab. Once there, you discover Alphys implanted Determination in a flower and Papyrus regrouped everyone with the help of "a tiny flower"…
- Upon reaching the palace, if you keep going past the throne room entrance and instead head down some stairs, you'll find a few coffins, one of which has whichever name you've input on it. The initial thought is that at least Asgore was kind enough to prepare your burial for you, but this line of thinking disappears when it's obvious that he has no idea who you are. This foreshadows the Fallen Child's actions and the twist involving them.
- Speech patterns can reveal a lot about an individual. For example, there are only two creatures in the underground who greet people with the word "Howdy": Asgore Dreemurr and Flowey. In hindsight, this becomes quite an obvious hint that Flowey is actually Asriel Dreemurr, Asgore's son. The one time Papyrus says "Howdy" is when he's secretly being led by Flowey.
- Additionally, two characters will greet you with the word "Greetings", Toriel and Chara.
- After realizing that you can't hurt each other, the Mad Dummy threatens to keep you trapped there by never ending the fight. Guess what Sans tries at the end of your fight with him in a Genocide Run?
- There are multiple subtle things in the True Labs that foreshadow the plot twist that Frisk is their own person, not just a player insert. At three different points, if you tell them to do something, they'll behave erratically or without your input. When you approach the shower, they move slower, showing they're afraid. When you try to laugh at the Snowdrake Amalgamate, they instead burst into tears (which is lampshaded, "what, you didn't do that?"). Lastly, after finishing each of the Dreemurr family home videos, they'll turn away from the TV like they know this isn't something they should be looking at.
- The theme that plays when you're sparing Asgore is Asriel's theme. Guess who's secretly in the room right now.
- In the last phase of the Neutral Route's final battle, the in-game music briefly reprises a portion of "Memory". This is a tune that is prominently featured in the song "Undertale" (which plays when the residents of New Home recount the life and death of Asriel and Chara), and turns out to be Asriel's theme, hinting at the True Pacifist Route's revelation that Flowey is a reincarnated Asriel.
- After beating the game for the first time and sparing Flowey, he will start giving you hints on how to get the Golden Ending. Pester him enough, and he will eventually tell you to stay away from "Smiley Trashbag" (aka Sans) for causing him 'more than his fair share of resets' before. Then you go for the No Mercy route and you find out that Flowey is being completely sincere…
- In Toriel's home, there are three different-sized chairs at the dining table. This first foreshadows that a family of three used to live here: Toriel, Asgore, and Asriel.
- Toriel's diary is a record of bad puns, with the one we see being skeleton-themed. In the True Pacifist ending, it's revealed that Toriel and Sans held a pun-based friendship from each side of the door to the outside, so the jokes written down are her remembering the puns she exchanged with him.
- Toriel's bedroom contains a vase of prominent yellow flowers, the first instance they are hinted to be connected to her family and son.
- The top half of the Delta Rune and the prophecy associated with it is theorizes that either a pure savior or a dark angel holds the solution to monster freedom. In the True Pacifist ending, both are true. The player is the savior who fights Asriel, the villain seeking monster freedom, who takes on an angelic form in his battle.
- In the news-report section of Hotland, Mettaton's blurbs give hints that all of the objects to choose to report on are bombs.
- If you continuously call Toriel after she gives you the cell phone and tells you to stay put (but not flirt with her), she'll suggest something for you to do, such as going to a pile of leaves and pretending to rule over it with an iron fist. This hints towards her frustration against King Asgore.
- A set of ancient glyphs in the Waterfall note that "Humans are unbelievably strong. It would take the SOUL of nearly every monster... ... just to equal the power of a single human SOUL." Sure enough, at the end of the True Pacifist route, Asriel is able to break the barrier, which is supposed to require seven human souls, without killing you by using the combined power of the six souls collected already, plus the souls of every single monster in the underground.
- When Gerson describes the Delta Rune, he pauses for a moment before remembering that it's the emblem of the Kingdom of Monsters. Almost as if there was another kingdom it'd be associated with...
- Mettaton first appears by crashing through a wall in Alphys's lab. Examining the hole in the wall reveals that it's shallow and doesn't have any passageways in or out, kind of like a very wide closet. It's another hint that Mettaton and Alphys were working together. The only way Mettaton could have gotten into that wall is if someone repaired the wall behind him.
- A monster in Grillby's mentions that due to the properties of the food in the Underground, the monsters are incapable of producing any kind of waste and thus don't need bathrooms.note You first meet Alphys as she's emerging from a door with a bathroom sign next to it, although given her otaku nature, it's presumed to be recreational. It's later revealed to be an elevator to the True Lab that she disguised as a bathroom from the outside.
- There are several locations where you can find cameras in Snowdin and Waterfall, though you typically have to go out of your way to find them. When you first encounter Alphys, she reveals the cameras were set up by her and she was watching you throughout your journey.
