An extraterrestrial or alien is a lifeform that did not originate on Earth. (The word extraterrestrial means 'outside Earth'.) Extraterrestrials are a common theme in modern science-fiction, and also appeared in much earlier works such as the second-century parody True History by Lucian of Samosata.
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.
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).