It starts right at the start, when Asriel finds Chara. But as time goes on Chara turns darker, and the timeline becomes broken but someone comes along to fix and restore a once peaceful timeline.
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.