Chapter 1: Betrayal Kill

"*if we're really friends.. you won't come back." - Sans

What if Frisk took this advice literally and quit out of the game right at the end of their genocide run without resetting, leaving the few survivors (namely Sans, Asgore, Alphys, and Flowey) to salvage whatever they can from the wreckage of the empty, ruined world?
What if they eventually do RESET, wiping everyone's memories, and then do the same genocide run all over again?

Can the determined demon be stopped?
Can the angel of death be SAVED?
And if so, who can pay the consequences for their sins?
Who would even be willing to?

This is the story of Undertale: Aftermath.