With “The Doctor Falls,” showrunner Steven Moffat caps off not one, but two story arcs which have run through the entirety of Doctor Who’s 10th season. One of these arcs has been so hidden in the background that its rising to the surface and resolution comes as a great surprise, while the other has been gradually developed throughout the 12 episodes. It all began as the mystery of what the Doctor (Peter Capaldi) and Nardole (Matt Lucas) were guarding in their secret vault in the basement of the university where they met Bill (Pearl Mackie). After we learned that the vault contained Missy (Michelle Gomez), the focus shifted to the Doctor’s attempts to rehabilitate her and resume their old friendship. The result of that effort was still in the balance when, in last week’s episode, they landed on a 400-mile-long spaceship containing both Cybermen (who captured and converted Bill into one of them) and Missy’s own prior incarnation, the Master (John Simm).