The Doctor asks Clara the same question, only using some things from all of their (Season 8) adventures. Semi-spoilers, and yes, I suck at summaries.
AU post 50th anniversairy, where a rouge Timelord manages to escape the Time War before the Doctor puts Gallifrey in a pocket universe,then, after changing his face, vows to seek vengeance and destroy the Doctor. Rated T for violence and possible adult themes and language.