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.
Regeneration (Doctor Who)
There is also inconsistency as to whether the Doctor can recognise his own future incarnations. In the original series, the Doctors usually have no difficulty recognising one another (in "The Three Doctors", the First, Second, and Third Doctors all know one another on sight. However, in "The Five Doctors", the First Doctor does not recognise the Fifth Doctor as himself and needs to be introduced by the Fifth as his fourth regeneration. In "Time Crash", after the TARDIS is at different points in its own timeline merge due to downed shields, the Fifth Doctor spends much of the mini-episode believing that the Tenth was merely a fan who had somehow gotten into his TARDIS, having somehow altered the 'desktop theme'; the Tenth Doctor constantly tries to convince his younger self that he is his future self, only doing so by using a predestination paradox to cancel out a universe-destroying black hole with a supernova by blowing up the TARDIS. When the Tenth and the Eleventh Doctor meet each other in "The Day of the Doctor," the Tenth Doctor appears to sceptically recognise his successor, yet he appears to be fully convinced only after both compare their respective sonic screwdrivers. The War Doctor on the other hand is initially completely oblivious to meet two of his future regenerations (initially mistaking them for future companions), again being convinced by their sonic screwdrivers. When the First Doctor meets the Twelfth Doctor, the First Doctor sees his future self as another Time Lord come to take back his TARDIS and requires convincing as to otherwise.