Well fucking damn?!

THIS IS KIND OF DISTRUBING
BUT I LIKE IT XD
I'm definitely faving this. I so enjoy reading everyone's theories on Gaster!
Keep writin' breh

Ok. Let me get this straight.

This is the experiment that kills Gaster. When he "falls into his creation." Gaster goes down into the Void, and he's got a long safety tether attached to him. It's got a mechanical winch, but Sans is there with telekinesis as a backup. So Gaster goes down, some sort of wave goes through the Void, and he starts getting "corrupted." In the process, the winch breaks. Sans is trying to hold Gaster up with the assistance of this mysterious "Natus" character, but the explosion (?) that broke the winch is also about to drop a room on Alphys, who is controlling all of this equipment. Sans wants to save her, but Gaster comes first. So Gaster keeps on getting corrupted, has some sort of pain in his head, and touches his face, messing up his eyes. This explains why his hands have holes and his eyes have cracks, I guess? So Gaster slowly dies, Sans drops the tether to save Alphys, Interna (Whoever the hell that is) is furious, but not as much as the Natus guy, who seems to be obsessed with Gaster. So he gets kicked out because he doesn't rank quite as high as the others, and our "Doctor Sans" has to face up to Interna. She says that she's going to resign if ASGORE doesn't punish Sans. Obviously, he doesn't, so she does. Meanwhile, Alphys is worried about the "Followers" and Sans says he and Papyrus are moving to Snowdin. Then edge.

Ok. So let's examine some things. First, we've got Sans and Alphys. There's some heavy implied Sansphys here, and Alphys even starts to say "You can't leave me" before she cuts herself off. But Sans seems to care more about duty than Alphys, at least until he decides that "The Doctor" is dead. Afterwards, he probably wants to leave his whole life behind, which maybe explains why he and Alphys know each other, but are awkward about it at the end of UT.

Then we've got Gaster himself. In this whole story, the name "Gaster" isn't used once. He's always just The Doctor. So we've got to assume one of two things. Either just nobody uses his real name, or maybe this is just what the title entails. A FLASHBACK. And maybe Sans is flashing back, but he doesn't remember the Doctor's name, so his mind just fills in the blanks. It's weird, but the best explanation, I think.

And Interna. Clearly her name is a play on "intern" (and not a very clever one), the question is, WHY? I mean, she's clearly equal in rank with Sans and Alphys, so maybe she just started as an intern. Anyway, it's strongly implied that she quits at the end of the story. So who's unaffiliated with anyone, knows about Gaster, and we don't know the identity of? Only one match. I think Interna is clearly supposed to be the Riverperson.

Then there's Natus. I wasn't sure about this guy, but a quick Google search says that "Natus" is Latin for "Theme." That didn't make sense, but turns out it can also mean "Born," "Son," "Made," "Formed by Nature," and "Made by Nature." With those translations used, it seems pretty clear to me. Some way or another, Doctor Gaster created Natus. Whether he was artificial, some kind of clone, or his biological son, I think Natus is the son of Gaster. What else explains his absolute worship of this guy? And what else explains why he was allowed in this super-important experiment when Sans specifically says that he's "not even supposed to BE here!"

So yeah. That's it. I just devoted like twenty minutes of my life and at least one Google search to writing a theory about this one random Undertale fanfic. But hey, it was pretty good. Esoteric as all hell, but pretty good.

By the way, spoiler alert. This post kind of explains the whole plot of the story.