Regeneration (Doctor Who) in the context of "The Deadly Assassin"

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.

Fourteen actors have portrayed the Doctor in a leading role in the British science fiction television series Doctor Who. The series' protagonist, the Doctor is an alien from a species called Time Lords. The transition to each succeeding actor is explained within the narrative through the plot device of regeneration, which allows Time Lords a change of cellular structure when they near death, changing their appearance and personality. The first actor to portray the Doctor was William Hartnell beginning with An Unearthly Child (1963) through The Tenth Planet (1966). In the final moments of the serial, the First Doctor regenerated into Patrick Troughton's Second Doctor.

The most recent incarnation, the Fifteenth Doctor, was played by Ncuti Gatwa, who took over the role from David Tennant following the conclusion of "The Giggle" (2023). Gatwa departed the role in "The Reality War" (2025).

The Fifteenth Doctor is an incarnation of the Doctor, the protagonist of the British science fiction television series Doctor Who. He is portrayed by Rwandan-Scottish actor Ncuti Gatwa in two series and two specials. As with previous incarnations of the Doctor, the character has also appeared in Doctor Who spin-offs, both during and after the character's televised appearances.

Within the series' narrative, the Doctor is a millennia-old alien Time Lord from the planet Gallifrey, with somewhat unknown origins, who travels in time and space in their TARDIS, frequently with companions. Usually, at the end of each incarnation's life, the Doctor regenerates; as a result, their physical appearance and personality changes. However, the Fifteenth Doctor emerged unconventionally, via "bi-generation", in which he split from his predecessor, the Fourteenth Doctor's body, rather than replacing him, allowing both incarnations to co-exist. In contrast to his predecessors, Gatwa portrays a Doctor who is attempting to let go of much of his recent emotional trauma, with a more emotionally balanced, liberated, fun-loving, and affectionate outward persona.