Companion (Doctor Who) in the context of "The Doctor (Doctor Who)"

The Doctor, sometimes known as Doctor Who, is the protagonist of the long-running BBC science fiction television series Doctor Who. An extraterrestrial Time Lord, the Doctor travels the universe in a time travelling spaceship called the TARDIS, often with companions. Since the show's inception in 1963, the character has been portrayed by fourteen lead actors. The transition to each succeeding actor is explained within the show's narrative through the plot device of regeneration, a biological function of Time Lords that allows a change of cellular structure and appearance with recovery following a mortal injury.

A number of other actors have played the character in stage and audio plays, as well as in various film and television productions. The Doctor has also been featured in films and a vast range of spin-off novels, audio dramas and comic strips.

Doctor Who is a British science fiction television series broadcast by the BBC since 1963. The series, created by Sydney Newman, C. E. Webber and Donald Wilson, depicts the adventures of an extraterrestrial being called the Doctor, part of a humanoid species called Time Lords. The Doctor travels in the universe and in time using a time travelling spaceship called the TARDIS, which externally appears as a British police box. While travelling, the Doctor works to save lives and liberate oppressed peoples by combating foes. The Doctor usually travels with companions.

Beginning with William Hartnell, fourteen actors have headlined the series as the Doctor; the most recent being Ncuti Gatwa, who portrayed the Fifteenth Doctor from 2023 to 2025. The transition between actors is written into the plot of the series with the concept of regeneration into a new incarnation, a plot device in which, when a Time Lord is fatally injured or weakened from old age, their cells regenerate and they are reincarnated into a different body with new mannerisms and behaviour but the same memories. This explains each actor's distinct portrayal, as they all represent different stages in the Doctor's life and, together, form a single lifetime with a single narrative. The time-travelling nature of the plot means that different incarnations of the Doctor occasionally meet. The Doctor can change ethnic appearance or gender; in 2017, Jodie Whittaker became the first woman cast in the lead role, and in 2023, Gatwa became the first black actor to lead the series. The different incarnations wear distinctive costumes; designers include Alexandra Tynan and Barbara Lane.