Doctor Who
Thirteen actors have headlined the series as the Doctor. The transition from one actor to another is written into the plot of the show with the concept of regeneration into a new incarnation, a plot device in which a Time Lord "transforms" into a new body when the current one is too badly harmed to heal normally. Each actor's portrayal is unique, but all represent stages in the life of the same character. Together, they form a single lifetime with a single narrative. The time-travelling feature of the plot means that different incarnations of the Doctor occasionally meet. The Doctor is currently portrayed by Jodie Whittaker, who took on the role after Peter Capaldi's exit in the 2017 Christmas special "Twice Upon a Time".
Twelfth Doctor
Capaldi has previously appeared playing other roles in the "Doctor Who" franchise. He portrayed Lobus Caecilius in the 2008 episode "The Fires of Pompeii" and John Frobisher in "", the 2009 serial of the "Doctor Who" spin-off "Torchwood". Moffat has stated that he plans on explaining over time why there are three characters in the "Doctor Who" universe with the same appearance; his predecessor Russell T Davies had once explained to him a theory for the first two, and upon Capaldi's casting assured Moffat that the explanation would still work. This situation was alluded to in "Deep Breath", when a confused Doctor is reminded of Caecilius when he examines his face in a mirror. In "The Girl Who Died", it is explained that the Doctor chose Caecilius' face to remind himself that he is the Doctor, and that he can always save people. While not explained in the episode, Moffat later said that he also shares a face with John Frobisher as Frobisher is Caecilius's descendant, and represents the end of Caecilius's bloodline, referencing the Doctor's "eternal battle with doom and destiny." Capaldi's casting marks the second time an actor has previously appeared in the series and then been cast as an incarnation of the Doctor, the first being Colin Baker.