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're a human who has fallen down into the underground. There you meet a certain skeletal scientist and his sons. Will you get close to the family or will you become their enemy. Loosely based on TheBombDiggity666 Dadster comics (which you should totally check out if you haven't already) and occurs before frisk's appearance. Also artwork is not mine~