The normal story of Undertale has gone on for a long time. Reset after rest. The resets never seemed to end. But finally, the code had started to dissolve under the pressure. The first reset after this started, it seemed fine. Bits and pieces are left out. A frame here, a pixel or two there but nothing much. Next reset, more is missing. The one after that; more. About after the sith reset. The dimensional barriers broke and other AU's started to seep through. A bleep there of some sans from a random AU here or there, etc... Each reset leading down an even more broken world each time. Personalities of characters are changed each reset and it changes at random.
Then, after one reset, it all seemed normal. Not a single blip or a frame missing. Then at the end of the game, when the human is just about fighting Asgore, black. The game sends the human to the beginning. Except, things are dark, creepy and broken more than ever.