The Doctor (Doctor Who)
The Doctor is the title character in the long-running BBC science fiction television programme "Doctor Who". Since the show's inception in 1963, the character has been portrayed by thirteen lead actors. In the programme, "the Doctor" is the alias assumed by a centuries-old alien—a Time Lord from the planet Gallifrey—who travels through space and time in the TARDIS, frequently with companions. The transition to each succeeding actor is explained within the show's narrative through the plot device of "regeneration", a biological function of the Time Lord race that allows a change of cellular structure and appearance with recovery following a potentially fatal injury.
Doctor Who
There have been instances of actors returning at later dates to reprise the role of their specific Doctor. In 1973's "The Three Doctors", William Hartnell and Patrick Troughton returned alongside Jon Pertwee. For 1983's "The Five Doctors", Troughton and Pertwee returned to star with Peter Davison, and Tom Baker appeared in previously unseen footage from the uncompleted "Shada" episode. For this episode, Richard Hurndall replaced William Hartnell. Patrick Troughton again returned in 1985's "The Two Doctors" with Colin Baker. In 2007, Peter Davison returned in the Children in Need short "Time Crash" alongside David Tennant, and in 2013's 50th anniversary special episode, "The Day of the Doctor", David Tennant's Tenth Doctor appeared alongside Matt Smith as the Eleventh Doctor and John Hurt as the War Doctor, as well as brief footage from all of the previous actors. In 2017, the First Doctor (this time portrayed by David Bradley) returned alongside Peter Capaldi in "The Doctor Falls" and "Twice Upon a Time" (both 2017). In "The Name of the Doctor" (2013), the Eleventh Doctor meets a previously unseen incarnation of himself, subsequently revealed to be the War Doctor. Additionally, multiple incarnations of the Doctor have met in various audio dramas and novels based on the television show.