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've heard the stories, of the mysteries of a man who guides lost ladies home by night legend says his voice is like a demon of hell and anyone who's ever approached him has died. One night you get lost and you uncover his real story