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.
History of Doctor Who
The seventh season, at twenty-five episodes, was shorter than any before, and established a pattern of "Doctor Who" seasons being between twenty and twenty-eight episodes in length, one that would last until the middle of the 1980s. However, although the new format of the Doctor being stuck on Earth had proved popular enough to save the programme from cancellation, neither Letts nor his script editor Terrance Dicks were particularly keen on the idea, and from the eighth season onwards sought reasons for the Doctor to be able to travel in time and space again, eventually having the Time Lords grant him full freedom at the conclusion of the 1973 tenth anniversary story, "The Three Doctors", a serial which also featured guest appearances from Troughton and Hartnell, the latter in a restricted role due to his poor health.