IDKTale is an AU that is a series of events that happens after Frisk's 100th reset, that causes everyone to forget not only what happened in the last timeline, but EVERYTHING. They forgot why they're Underground, they forgot how anything in the Underground works, they even forgot EACH OTHER. (Yes, even Sans and Papyrus forgot who the other is.) The only ones who remember are Frisk, Flowey, Chara, and Gaster. Now Frisk must try to help all of the monsters regain their memories.
Frisk, now overcome with grief seeing her (and yes they are a girl in this AU) friends having no memory of each other, will try to help them in any way she can to learn how to function Underground, and hopefully will remember each other.
Flowey reluctantly helps Frisk in this AU to help the monsters regain their memories. The reason he gives is:
"Its no fun to torture them if there's no one they care about"
Gaster becomes an antagonist in this AU, as he tried to steal Frisk's soul for the 'greater good' as a SINGLE more reset will cause the timeline to collapse.
Chara is even more adamant to kill all of the monsters in this AU, because they are more ignorant and vulnerable then they have ever been before.