Undertale | Timeswap AU

TIMESWAP AU

Basically Sans is very good at timetravel. With along of his amazing teleportation trick and time jumps, he can't escape the wrath of Chara or the fear of everything being reset, so he jumps back in time to see his little Pap again from when he was the purest and most innocent. Basically the only thing that keeps him alive. He doesn't know the consequenc of his time jumps. Everytime he does he swaps times with the sans from where he goes. So... He puts a younger sans into the future.

so the younger sans doesn't understand what's going on and thinks he's dreaming but is starting to catch up to what is going on and keeps it silent, acting like nothing is changing, just going with what his slightly grown older brother (that he hasn't seen before) 

JUST A VERY DIFFICULT THING TO EXPLAIN BUT SANS IS JUST VERY EMOTIONALLY MESSED UP