Papyrus has always been a kind of mysterious monster. He just appeared at the mansion, no questions asked. No one has ever asked him about his past, his family, his life before the mansion.
So when people finally start asking these questions, Papyrus decides it's time to explain what's going on, and shares a secret that no one, not even Korey, would know about that led him to become the monster he is today.
(Note: in this story, Gaster is seen as a kind father to Papyrus. This is not what actually happens in UM cannon, in my eyes.
NOTHING IN THIS STORY IS TRUE)
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.