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.
The Doctor (Doctor Who)
Due to time travel, it is possible for the Doctor's various incarnations to encounter and interact with each other, although supposedly prohibited by the First Law of Time (as stated in "The Three Doctors") or permitted only in the "gravest of emergencies" ("The Five Doctors"). In the 1963–1989 television programme, such encounters were seen on three occasions: in "The Three Doctors" (1972), "The Five Doctors" (1983) and "The Two Doctors" (1985). In "Day of the Daleks" (1972), the Third Doctor and Jo Grant very briefly met their future selves due to a glitch during a temporal experiment (the serial was supposed to end with the same scene depicted from the perspective of the "other" Doctor and Jo, but was excised because it was anticlimactic). In "Father's Day" (2005), the Ninth Doctor and Rose observed but did not interact with past versions of themselves; when Rose changed history, the earlier selves – after momentarily noticing Rose running past – vanished and a temporal paradox was created that attracted the extra-dimensional Reapers. The Tenth and Fifth Doctors met in the TARDIS in the mini-episode "Time Crash", which aired on 16 November 2007 as part of the BBC's annual "Children in Need" appeal. This marks the first time the Doctor has met a previous incarnation since the programme's revival. Although the scene aired outside the programme itself, it was established as taking place between the events of "Last of the Time Lords" and "Voyage of the Damned".