Tonight’s Doctor Who is a turning point for the season. After a run of standalone adventures for the Doctor (Peter Capaldi) and Bill (Pearl Mackie), by the end of “Extremis” we’re suddenly in the middle of a big multi-part tale. It feels quite similar to the way 2011’s mid-season finale, “A Good Man Goes to War,” finally resolved the long-running mystery of River Song’s identity, only for that resolution to send the season’s plot arc off in a whole new direction. Ever since “The Pilot,” we’ve been teased with the riddle of who or what is in the secret vault that the Doctor and Nardole (Matt Lucas) have been guarding, and why the Doctor took up this duty. “Extremis” fills in the answers to those questions, but it’s clearly only the prologue to a much larger story.