Frisk freed all the monsters. Everyone lived happily ever after. Everything was wonderful.
Then Chara finds her way back. Suddenly the world has restarted and poor Frisk is left with a murderer in their head and has to watch while the world they had come to love ends.
There will be a happy ending, I promise, but it will come after a LOT of angst. Please read only if you are comfortable with all the tags, it will get bad before it gets better.