Pretty self explanatory. This takes place after the Pacifist route in Undertale. Through determination, Frisk was able to bring back Chara, Asriel (Asriel is Flowey per say), and Gaster. I believe that Gaster is the father of Sans and Papyrus. I also don't believe that Chara is evil. The monsters are on the surface, and although they are amongst humans, most humans have accepted them.

You manage to escape a facility where you had been experimented on. But will you be forever free? Will things go as smoothly as you hoped?

Reader has fallen into the Underground. They live with the famous skelebros, but a certain one of them won't fall for their facade.