A Revised (not so) Brief Study Into The Nature Of Deltarune
Disclaimer: This is a revised version of a theory I came up with back in 2022, having played just the first two chapters. Now I'll take into account chapters 1 to 4.
TL;DR:
For some time I've been quite intrigued by the appearance of some very particular leitmotifs that Undertale's and Deltarune's OSTs have in common. This took me down a rabbit hole that brought me to the realization that it's quite possible that DR Sans and UT Sans are one in the same, and the Reset Theory on Deltarune is probably right.
This forces me to believe that Deltarune is a prequel from the perspective of the characters, and a spiritual sequel from the perspective of the Soul/Player.
The Pre-Sequel Theory
First and foremost, pardon my English. It's not my first language and sometimes I mess up.
Secondly, this is a LONG post. There's a lot –a LOT– of text and no images at all to ease the reading. I advice jumping into it only if you don't have anything better to do for a bit. The PDF I first wrote this on is 6 and a half pages long.
Index:
Motivations.
Clues in Undertale.
Clues in Deltarune.
The Delta Rune.
Blood.
Conclusions.
Problems with the theory.
1 Motivations:
I've always been fascinated by soundtracks and ever since Undertale came up, its soundtrack wasn't an exception.
More recently I've been a bit more focused on the leitmotifs that Toby Fox uses in both Undertale and Deltarune. As many people noticed, they share a lot of common themes. For many of them it's easy to understand why (for instance, for common character's themes, like Sans', or the theme for Justice) but for some others it's not that simple.
Assuming that this commonality in themes was proof of a direct relationship between the games, I tried to study its nature and what elements outside the soundtrack can be used as evidence of this connection. This brought me to Sans ,because of course it did.
Trying to validate these assumptions took me down the rabbit hole that theory crafting around Sans has always been, and the musical side of my reasoning ended up being but as small part of the whole proofing. Regardless, I'm quite happy with the result, albeit it's not free of problems that I'll address and discuss at the end of the post.
With that out of the way, let's begin by talking about small details that Undertale gives us, and how it hints us that its past is somehow connected to Deltarune:
2 Clues in Undertale:
There’s a lot of non-music related hints that point towards a possible connection between both games in Undertale. Here's a summary of all the hints found within this game:
To begin with, there's what we know about Sans' backstory:
Both him and Papyrus appeared one day in Snowdin, seemenly out of nowhere.
In Sans’ lab there’s a broken machine and a poorly drawn picture of 3 people we don’t recognize, with the words ”don’t forget” written on it.
Sans insists a lot about about how he gave up trying to go back (somewhere, sometime, or both).
On the other hand, there’s the famous Gaster’s Entry 17, that could imply some knowledge of the Dark Fountains by the former Royal Scientist. Another Gaster related piece of evidence is goner Clam Girl. Soon before the release of Deltarune Chapter 1, in the Switch version of the game, Clam Girl would turn into a goner and say that the time to meet this Suzy ”... is fast approaching”, before disappearing with the sound Mystery Man makes when disappearing himself. Although Suzy is not written the same way as Susie, there’s no reason to believe that Clam Girl is not talking about Deltarune’s Susie. This not only hints to Undertale and Deltarune being connected, but to Gaster being behind the whole connection, since she turns into a goner sprite and disappears the same way Mystery Man does. I know Mystery Man is not confirmed to be Gaster, but it’s definitely related to him in someway.
There must be something going on with Papyrus as well, since he’s Sans’ brother and comes to Snowdin with him, but in Undertale he doesn’t seem to be as knowledgable as Sans. Since Papyrus doesn't bleed, as does Sans, maybe they're different in some way, and don't share as big a past as previously thought.
2.1 Sans’ lines during combat:
During the fight against Sans, nearing the end of the Genocide Route, we get some snippets of his backstory within his lines of dialogue. He begins, after the Player’s first attack, by saying:
our reports showed a massive anomaly in the timespace continuum. timelines jumping left and right, stopping and starting...until suddenly, everything ends. heh heh heh... that's your fault, isn't it?
We don’t know who is he working with, but it seems they know very well about the power to control the timeline, since they were monitoring it.
Sans continues with:
knowing that one day, without any warning... it's all going to be reset.
look. i gave up trying to go back a long time ago. and getting to the surface doesn't really appeal anymore, either. cause even if we do...we'll just end up right back here, without any memory of it, right?
to be blunt...it makes it kind of hard to give it my all.
In these lines we get a bit more information. Sans is –understandably– depressed about the whole reset situation, but mentions ”going back” to somewhere. We know it’s not somewhen because he doesn’t remember earlier timelines –besides the occasional déjà vu– and it’s not the Surface, since he mentions it right after that line. So, where else would Sans be trying to go back to? It can’t be anywhere in Undertale's universe, since he can move freely through the Underground –even through teleportation– and he specifically states that it’s not the Surface. Thus, it can only mean somewhere outside this universe.
A bit after that he continues with:
all i know is... seeing what comes next... i can't afford not to care anymore.
What does he mean by ”seeing what comes next”? He doesn’t know the future, nor what the Player did in other timelines. So, what is he talking about? Back in 2022 I had no clue what he could be referring to. Now I'm inclined to think of the Roaring, or the End of the Prophecy.
2.2 Sans’ secret lab:
After Sans gives the Silver Key –which happens to be a reference to a book about timetravel by H.P. Lovecraft– to the Player we can access his Secret Lab, hidden underneath his house. Inside of it we find:
A covered up broken machine.
Blueprints for a machine written in un- readable symbols.
A badge.
A photo album with:
A picture of Sans, happy, with other people the Player doesn’t recognize.
If the Player already fought Asriel: A second picture appears, showing Sans, Frisk and their friends.
After the v1.001 patch of the game, if the Player has talked to Clam Girl and knows of Suzy, from the back of the photo album sticks out a poorly drawn picture of three smiling people with the words ”Don’t Forget” written on it.
The first three items, although interesting, don’t give us much to work with. The machine is covered up and the blueprints are unreadable. The badge could become another huge hint in the future, if during Deltarune’s festival Sans happens to win a badge for winning at something. But this is purely speculation so I won’t be taking it into account –for now–.
The pictures and the drawing, on the contrary, give us much more information. Let's discuss them a bit more in depth:
2.2.1 The photo album:
By the time the Player gets the Silver Key, possibly after the battle with Asriel, they (and Frisk) have met every main character in Undertale’s story. Thus, the people in the first picture, who neither the Player nor Frisk recognize, can’t be anyone we know. They could be random people from other places in the Underground Frisk hasn’t met –although, from a storytelling perspective, this doesn’t make much sense– or they could be from Deltarune's universe -which makes a lot more sense storywise-.
Furthermore, there’s the poorly drawn picture. Since this picture only appears after talking to goner Clam Girl, a character heavily tied to Deltarune -specifically, to one Deltarune Character-, and it has the words ”Don’t forget” –words that don't mean anything special in Undertale, but are extremely important an present in Deltarune from the very beginning– written on it, it’s safe to assume that the three poorly drawn characters are a trio from Deltarune. Due to Frisk not recognizing them, they have to be characters that only appear on Deltarune. What trio of characters do we know from Deltarune that fit these characteristics? I’m fairly sure Kris and Susie are two of the three, but the third is a bit more complicated. My first suspect was that it was Ralsei but, since he’s a darkner and he can't go into the lightworld, it can't be him. My other candidate are either Noelle or Dess, since Berdly dies in the Snowgrave route of Chapter 2 –and Toby Fox likes to keep all possible endings as canon– and we don’t know of any other important lightners that don't appear in Undertale as well.
2.3 Darker, yet darker:
Gaster's Entry 17 states:
ENTRY NUMBER 17, DARK, DARKER, YET DARKER, THE DARKNESS KEEPS GROWING, THE SHADOWS CUTTING DEEPER, PHOTON READINGS… NEGATIVE THIS NEXT EXPERIMENT, SEEMS VERY, VERY INTERESTING… WHAT DO YOU TWO THINK?
At the beginning of Chapter 3, Ralsei explains to Susie that the Dark World is what happens to reality in the total abstence of light. When it's darker than dark. Darker, yet darker. So dark you could take away the light that wasn't there: photon readings... negative.
It's now that we can say, with absolute certainty, that Entry 17 talks about opening a Dark Fountain within Undertale's Universe. And Gaster, to whom we talk to outside the Deltarune chapters, is responsible for it.
I don't really like to get into Gaster while theory crafting. Sans is enough of a controversial rabbit hole for me. But after Ralsei's exposition in Chapter 3, I can't look the other way.
The connections between WD Gaster and Sans run deep. Not only are they 2 out of the 3 characters whose speech is characterised by the font that names them, but UT Sans' infamous attack is called "Gaster Blaster". Furthermore, it's known that Sans and Alphys shared a past, and Gaster could be this missing link between the two of them. Could it be that Gaster is referring to them at the end of Entry 17? Hard to say, and we can only speculate. But it would make sense that Sans, if he had come from Deltarune's Universe, would try to study Dark Fountains with the Royal Scientist and his pupil, as well as the passage of time, as evidenced by his comments during the fight against him at the end of Undertale's Genocide Route. It would also explain what he meant by "i gave up trying to go back a long time ago". He gave up researching the Dark Fountains after Gaster was consumed by the experiment and scattered through time and space.
3 Clues in Deltarune:
Back in 2022 I had only played Chapters 1 and 2, a quarter of the game. Even though this wasn't much content to work with, the leitmotifs Toby Fox used gave us a lot of information.
3.1 A brief introduction to leitmotifs:
*This part is skippable if you know what a leitmotif is*
For those who don’t know, a leitmotif is a small piece of melody that appears many times in a composition, and is associated to a character, a place, or an idea. Examples of this are the main themes of characters: a simple, recognizable melody that is associated to a specific character. For instance, the main melody in Nyeh Heh Heh!, which would be Papyrus’ leitmotif, or the common themes in Spear of Justice and Hammer of Justice, which would be Justice's leitmotif.
Soundtracks are rigged with leitmotifs, and neither Undertale’s nor Deltarune’s soundtracks are an exception. Since each one is associated to a specific character, place or idea, the use of the same ones in both games is something to keep in mind.
3.2 Leitmotifs that appear in both games:
The first song that plays inside Deltarune's , in Chapter 1, is Beginning, which has 2 leitmotifs in it: Once upon a time –from Undertale–, and Don’t Forget, –from Deltarune–. This second leitmotif appears in several other songs in all chapters.
The song that plays in the menu after you finish Chapter 1 is called Before the Story, which has the exact same melody as in Once upon a time. This leitmotif is one of the most –if not he most– used in Undertale’s soundtrack. For me, it’s kind of the ”Undertale theme”, for how much it appears in the game –same as Don't Forget would be Deltarune's theme–.
There are some other Undertale leitmotifs in Deltarune’s OST that I won’t be taking into consideration. These are the character’s leitmotifs, like Sans’, and other that represent important emotions like Determination, Justice, Despair and Hope –These leitmotifs are a bit difficult to recognize if you’re not deep into the music side of the games. Determination can be heard, among others, in Undertale’s Determination and Deltarune’s Rude Buster; Despair corresponds to the melody in Undertale’s Burn in Despair, and appears in Your Best Nightmare, Vs. Susie, at Queen’s basement, right before the Spamton NEO fight, and again in Chapter 3's Bit Roots; Justice is heard in Undertale's Spear of Justice and in Deltarune's Hammer of Justice, and Hope is sometimes called ”Snowdin’s theme”, which is used in Undertale’s Snowdin Town and Hopes and Dreams (thus why I prefer to call it Hope’s leitmotif) and in Deltarune’s A Town Called Hometown–. These leitmotifs being in both games is to be expected, since they either represent common characters or common emotions to both games, and Toby likes to reuse leitmotifs from old projects, so they're not really relevant to the theory.
Going back to the theory, the use of the words ”Don’t Forget” as a title for the most present leitmotif in Deltarune is a bit on the nose, taking into account the poorly drawn picture in Sans’ Secret Lab. The soundtrack acts as a constant reminder to not forget –pun intended– that drawing. But this isn’t the only insinuation Toby left in the game’s music. There’s another, much obscurer, clue: the name of the song Before the Story. Since it plays at the menu only after you play Chapter 1, it can’t be referring to ”before the story” of Deltarune, since that story has already begun. So, what if the Story is Undertale’s? If so, since it uses the most important leitmotif of Undertale in a song called ”Before the Story”, this would imply that Deltarune’s story happens before Undertale's.
Back in 2022 this was enough proof for me to be convinced of this theory. And yet, in 2025, it rained.
3.2.1 It's Raining Somewhere Else.
What is there to say about this. What is there to add. In 2015 Toby told us that It was Raining Somewhere Else. Ten years later he showed us where.
In Undertale this song is directly related to UT Sans' relationship with UT Toriel, and how this friendship is the reason he helps us throught the Underworld. In Deltarune, however, this songs plays after we leave the Church at the end of Chapter 4, on the way home. Nevertheless, it's back home that we learn of the friendship DR and DR Toriel have.
Not only the titles, not only the melodies –althought in Deltarune it sounds distant, almost like an old memory brought back by the smell of the rain–, but the context of these two songs connects them between games. Moreover, a slowed down version of the song can be heard at UT Sans' Secret Lab, as if the "Don't Forget" message inside it wasn't enough of a giveaway.
3.3 Non-musical clues:
How the Player interacts with the world seems to also imply a connection between games. Specially those lines that point to the Player knowing characters –or things about characters– that Kris doesn't know yet. For example, the first time we meet DT Sans there's the option to tell him that we're happy to see him again, something that makes sense for a Player that has already played Undertale, but doesn't for two characters who just met, as Sans points out.
This in and of itself is not enough to prove that the games are both a direct sequel and prequel of each other but, in combination with everything else, it heavily add to that idea.
Besides these interactions, which could be taken just as silly easter eggs, like the annoying dog appearances, there's another non-musical clue which brings us back to UT Sans: the Warp Doors, which look just like UT Sans' bedroom door, fire animation and all. What's more, Mystery Man's door in Undertale uses the same sprite as well.
I'm certain that Undertale's Underworld is not a Dark World, since there are no Dark Fountain to be found, nor darkners, the style of the menu is the same as in Deltarune's Light World, and to break The Barrier you need 7 souls, not just one. This complicates things, for the Warp Doors only appear inside Dark Worlds in Deltarune. But then again, in Deltarune, monsters seem to only possess magic powers inside Dark Worlds, while in Undertale all of them have this gift.
4 The Delta Rune:
Both the appearance of the Delta Rune in the two games and its name are big indicators of the connection between games. Even though in Undertale it’s not well known what its original meaning was, there’s a prophecy that sounds remarkably alike to that of the Delta Rune Legend from Deltarune. In Undertale there are two interpretations: either the Angel will murder all monsters, or free them all from the Underground. Additionally, in Deltarune, the Legend Ralsei tells us says that the Angel is to be defeated by three heroes: two lightners -a human and a monster- and a darkner.
Since the Delta Rune is older than written history in Undertale and it’s original meaning was lost to time, that one of the 2 interpretations for this symbol is so close to the Legend Ralsei gives us, with an evil Angel instead of a benevolent one, could mean that, originally, both Runes had the same meaning behind.
5 Blood & Magic:
Back in 2022 this part of the post began with:
TL;DR: As always, blood seems to be the biggest issue around Sans’ theories. To summarize, unless we get undeniable proof that DR monsters do bleed in some way before turning into dust, UT Sans bleeding could be a huge weak point for this theory.
Welp. We got it. Susie bled.
It’s fair to say that UT Sans does bleed in Undertale's Genocide Route, before turning into dust outside the screen. And he’s the only Undertale monster that does. We know he bleeds because he does it from the mouth, as well as the ribs, far from where the Player cuts him, which would only happen with regular bleeding around a digestive system rather than with ketchup stored within his rib cage.
Three years ago this was a huge nuisance, but now, knowing that a Deltarune monster can and does bleed, it' becomes another huge piece of evidence for this theory.
However, there's an issue. At the end of Chapter 1 a monster –not The Monster Kid– asks Kris if it hurts to be made out of blood, implying that other monsters don't bleed. This looks to be in direct contradiction to what we see Susie say (i.e. "Everybody bleeds") and do. As of now, I've no idea how to explain this line of dialogue.
5.1 Magic:
Deltarune monsters having magic powers is a highly debated topic. Nevertheless, Sans does teleport in Chapter 4, appearing both at his shop and at the Cattenheimer's home. Furthermore, it's implied that Catti holds some magical powers, which makes it all the more interesting that Sans teleports to her front yard.
6 Conclusions:
With all this in mind, I’m quite sure that, at the end of Deltarune, Sans will end up being stuck in the Undertale universe, possibly using the machine he keeps covered up in his lab. Since Papyrus doesn’t seem to be much different to any other monster in the Underground I don’t think he comes from Deltarune’s universe, so I fear a tragic end for that Papyrus, specially after his dissappearance after Chapter 3.
This ending would make sense of the whole ”Your choices don’t matter” thing. The Weird Route seemed to contradict this statement but, if just Sans is going to Undertale, whatever you do on Deltarune’s Universe won’t matter to Sans’ end. Undertale’s story is already written and the end we got will remain untouched, as Toby said. It also implies that our actions don't matter to Deltarune's end either.
We now know that the End of the Prophecy is a tragic one, as evidenced by Ralsei being terrified of what's to come, and Susie being enraged and in denial. A tragic end would also align with Sans' depression in Undertale, and him being the only one to come from Deltarnue's universe.
But, if Deltarune is a prequel, how is it that the Player recognizes the characters that both games have in common, and it’s advised that the game is played after Undertale? I believe that Deltarune is a prequel from Sans’ perspective, and a spiritual sequel from the Player’s. The Player, being the meta-being that it is in both games, doesn’t need to abide by the same temporal rules the characters are forced to follow. Thus, there's no contradiction in Deltarune being a prequel to Undertale. A Pre-Sequel.
7 Problems with this theory:
UT Papyrus seems to not know anything about his brother’s possible past in another universe, and acts as if they had been together since forever. They both appeared one day in Snowdin and his past seems as mysterious as Sans’, but they can’t be directly related since Papyrus doesn’t bleed.
There's a contradiction between Susie bleeding and the monster kid asking Krist if it hurts to be made out of blood.
I don’t know how will Sans get to Undertale’s world (my guess is Gaster, but he’s a bit of an easy-way-out in theory crafting).
Grillby’s. Sans’ convenience store, ’Sans, appears to have the same façade as Grillby’s in Undertale, with the name of Grillby rubbed off. This small detail is a possible proof against the sequel thesis, albeit it could just mean that Hometown had a Grillby’s before Sans’ convenience store and it closed down for some unknown reason.
Although some problems still remain, the list of issues with this theory has been greatly reduced after playing chapters 3 and 4. Furthermore, we now have great new pieces of evidence that point directly towards a connection between games.
