A human fell into the Underground, and began befriending every monster they came across, including the skeleton brothers. Their acts of kindness seemed to be genuine, giving the monsters no reason to distrust them... Until the human pulled a one-eighty and killed Sans when everyone least expected it.
This is the story of how Papyrus grieved over his older brother's death, how he discovered the truth about his mysterious sibling... and eventually, how he decides to avenge his sibling's murder.
But in the end he'll forget everything once the timeline resets again, right? That's what he's afraid of.
(Based on the AU "Disbelief Papyrus," but with an original twist. There are no pairings except for what's canon. This is mainly Papyrus-centric, but it does focus on mystery of Sans during the story.)