The Doctor is an 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 Tenth Doctor is an incarnation of the Doctor , the protagonist of the BBC science fiction television programme Doctor Who , who is played by David Tennant in three series as well as nine specials . The Doctor (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 .