The Doctor is a fictional alien.
Sarah Jane Smith is a fictional character played by Elisabeth Sladen in the long-running BBC Television science fiction series Doctor Who and two of its spin-offs . The Doctor is the title character and protagonist in the long-running BBC science fiction television programme Doctor Who . 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 his TARDIS , frequently with companions . The character has been generally well received by the public , with his enduring popularity leading The Daily Telegraph to dub him `` Britain 's favourite alien '' . The Time Lords are a fictional , ancient extraterrestrial people in the British science fiction television series Doctor Who , of which the series ' protagonist , the Doctor , is a member .