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)
In "Journey's End", the Tenth Doctor manages to avert his own regeneration by using some of the energy to heal himself, then channeling the remaining energy into his severed hand, thus retaining his appearance and personality. That regenerative energy was a key point in a "human–Time Lord biological metacrisis" inadvertently caused by Donna Noble that creates the Meta-Crisis Doctor while she obtains a Time Lord intellect. Later in the series, during the events of "The Time of the Doctor" the Eleventh Doctor revealed that it was considered a full regeneration; he just kept the same face due to "vanity issues", and that he was now in his final (13th) life (given that the Tenth Doctor counted as two regenerations and the revelation of the existence of the War Doctor, this made a total of 12 regenerations). However, during the same episode, the Doctor is given a new cycle of regenerations by the Time Lords, allowing him to regenerate for the thirteenth time into the Twelfth Doctor, with the Twelfth Doctor ("Kill the Moon") and Rassilon ("Hell Bent") each expressing uncertainty about how many regenerations the Doctor now has.