Shattered

Everything in the Underground goes into a continuous loop, even if only two monsters actually realize it. Those two are use to Frisk falling down (or sometimes the other one) then everything resetting before long and the cycle just repeats. That is until the day that Frisk isn't the one to fall down to the monsters.

Rive is a mystery to Flowey and Sans. Of course they're confused! After all this time of only having to deal with one kid and knowing what's bound to happen. Now, they're going in blind.

(Main character isn't Frisk, obviously)