About the AU
Aftertale's backstory is where Chara has full control of Frisk's body and keeps resetting the timeline and doing the genocide route for what Chara calls "research". After thousands of resetsSans feels like the only to beat Chara is through injecting determination in himself just before Chara fights him.
After 536 tries Chara finally was able to hit Sans. As he was dying, Sans walks off and notices he had the ability to CONTINUE, RESET and SAVE. Sans had also the ability to teleport to the save screen where he reset his timeline.
Sans (now nicknamed Geno because of the genocide route) cannot leave the save screen or he will turn to dust. Chara then gains so much determination that Frisk permamently looses control of their body and joins Geno!Sans in the save screen.
The current plot is that Geno tries to convince the original Sans to bring Chara to him in the save screen so he trap them there, erase the timeline forever, and keep them in a coma to stop them from restarting the timeline and killing anyone ever again. Subplots include W. D. Gaster attempting to convince Geno to "give up" and join him in the void.