Welp, finally finished a normal/pacifist run of Deltarune’s newest chapters, so I guess I’m posting Sans Undertale theories in this wretched year of our lord 2025.
I’m rusty on the wild fan theorizing of ye olden days, but I remember there being something about Sans being from either another timeline or another world. I think it was mostly connected to the stuff you can find in his basement, some dialogue from the pacifist and genocide endings (stuff about how he “can never go back” and “will never see them again” - which granted, could be connected to him remembering past resets), and dialogue from the Snowdin shopkeeper that said the skeleton bros “just showed up one day” and not clarifying from where. This Reddit post includes a lot of the speculation I remember from the time.
There was also a theory that Gaster was based on a cut character called “Grandpa Semi” from an earlier Undertale concept, who might’ve then been replaced by Alphys. The idea was that in Undertale’s metafiction, a cut character is kind of like writing a character out of reality, as it’s suggested might have happened to Gaster (depending on how you interpret Goner Kid’s dialogue). Undertale is written essentially as a game that knows it’s a game, with its gameplay functions existing outright in the fictional world, so I could see something like this being the inspiration for all the Gaster/mystery man secrets.
So if we entertain this as a possible inspiration for Gaster, we could apply a similar meta interpretation to Sans and Papyrus. Back in the before-times when Deltarune Chapter 1 was released out of nowhere, Toby said that he’d originally been working on concepts for Deltarune when he eventually switched over into creating what would become Undertale. So Deltarune (and specifically its ending) was an early concept in this project, as were the characters of Sans and Papyrus. I don’t know if they were originally made for Deltarune or if Toby came up with them after switching over to Undertale, but if they were made for Deltarune originally, how could that be translated in-fiction considering the paratextual and metatextual relationship between Deltarune and Undertale?
Well, maybe Sans and Papyrus were originally from Deltarune’s world. Maybe they somehow ended up in Undertale through reality-breaking (game-breaking?) means. Maybe this is somehow connected to Gaster, who it’s theorized is related to them (he has weird dialogue font stuff going on as well). Maybe it’s a form of time-travel, or they jumped into an alternate reality.
If this very speculative idea were true, I’m not sure what the order of events would be. Is the new Sans Deltarune the OG Sans before he isekai’s into Undertale? Or from Sans’s perspective, did he isekai from his original world into Undertale, and now has isekai’d into a similar-but-inevitably-different version of his original world? Is this pre-Undertale Sans or Post-Undertale Sans? (or is it just Deltarune Sans and I’m a fool)
Unless there are some scenes I haven’t unlocked (I haven’t done the weird route or finished the secret bosses yet), we don’t see that much of Sans in Deltarune, and infuriatingly nothing of Papyrus. Mostly Sans is a mischievous new guy in town who’s ““befriending””” your mom, and I’ve only seen him possibly teleport once (when he’s fucking around with his signs while simultaneously making “hotdogs” in the next room). His store is super suss since it seems to be the only building asset from Undertale in town - aside from, of course, his house. Did he bring his house and Grillby’s bar to this world somehow? (Or maybe Grillby just had the same bar here, sold it to Sans, and Sans did barely any renovating as is his way.)
Also, the song “It’s Raining Somewhere Else”? That plays when Sans takes you to dinner and is super spooky?? And an echoey, far-off version of it plays at the end of Chapter 4??? And I was sitting there thinking “oh shit it was raining is this the ‘somewhere else’” AND THEN I LOOK UP THE SONG AND IT’S CALLED “THE PLACE WHERE IT RAINED” TOBY I SWEAR TO GOD-
Anyway, the other main thing I’m questioning is Sans’s door being present in the Dark World - it was always a suss door. Was it always the way he took shortcuts in Undertale? Did he bring it there with him from Deltarune somehow? Who can say.
For all I know this has already been disproven by alternate content in the game, but I dunno. Playing has been taking me back to the less-on-fire days of yore, when I absorbed tons of Sans theories instead of worrying about impending US fascism. All I can say is I’m watching you bone-man, you can’t hide from me. 👈👀
I will add that Susie creating her own Dark World in Chapter 4 really sets my brain on fire in terms of relating Deltarune to Undertale. Her Dark World being just slightly different from the one that existed before has to relate to Deltarune’s relationship with Undertale thematically, if nothing else. I’m really excited to see where all this goes, and also terrified of what things will look like in 3+ years if/when we see the ending. Hopefully a worker’s revolution, or getting closer to it. I guess like Susie, you gotta hope, even if the odds seem against you. (i’m not projecting onto the themes of deltarune UR projecting-)
