The Doctor is the last of an ancient, nearly God-like race called the Time Lords from the planet Gallifrey. He fled his home world with his granddaughter, Susan, to explore the universe. He travels in a living time machine known as the TARDIS (Time and Relative Dimension in Space), which he stole from the Time Lords and which appears like a small London police box on the outside but which has nearly infinite dimensions on the inside. After some initial adventures with Susan in 1963 London, their odd lifestyle attracted the attention of two schoolteachers who forced their way into the TARDIS. The Doctor, wishing to keep secret his and Susan's existence, essentially kidnapped the teachers and took them on a number of adventures, eventually returning them and traveling with other companions, including, more recently, Rose Tyler, Amy Pond, River Song, Martha Jones and Donna Noble.
He is described as the universes greatest defender and has saved it countless times from all manner of alien, war or other disaster. He shows a soft spot for humans.
Throughout the series, The Doctor has been forced to regenerate ten times, a process he can undergo when he is near death and which causes him to change appearance, but retain his memories (explaining the 11 doctors and actors who have played him). He has been hunted by his own kind, the Time Lords, but his main enemies are the alien Dalek, from the planet Skaro; the Cybermen and his nemesis, The Master.
Background: Specifics about The Doctor, including his name and other details, are shrouded in secrecy within the world of the show; only Samantha Jones, River Song and possibly Clara Oswald are believed to know it. According to Dorium Maldovar and the Silence, the oldest and most dangerous question in the universe was "Doctor Who?" The Doctor's true name was apparently the answer, and if it is answered the Doctor's timeline could be accessed and corrupted, which could in turn lead to the destruction of the universe by reversing all of the times the Doctor has saved it.
He has displayed some medical knowledge and been described as a doctor of everything.
The Doctor is at least 900 years old. It has been said he can regenerate only 12 times, for 13 lives, but since this limit was set by the Time Lords, and because the series is now on its eleventh Doctor, this number is now seen as flexible. The Doctor himself once joked he could regenerate 507 times. He has two hearts, and a respiratory bypass system which allows him to go without air for a short while. His relationship with his traveling companions is general platonic, with two notable exceptions.
The Eleven Doctors are:First Doctor
(portrayed by William Hartnell
; 1963 - 1966)Second Doctor
(portrayed by Patrick Troughton
; 1966 - 1969)Third Doctor
(portrayed by Jon Pertwee
; 1970 - 1974)Fourth Doctor
(portrayed by Tom Baker
; 1974 - 1981)Fifth Doctor
(portrayed by Peter Davison
; 1981 - 1984)Sixth Doctor
(portrayed by Colin Baker
; 1984 - 1986)Seventh Doctor
(portrayed by Sylvester McCoy
; 1987 - 1989, 1996)Eighth Doctor
(portrayed by Paul McGann
; 1996)Ninth Doctor
(portrayed by Christopher Eccleston
; 2005)Tenth Doctor
(portrayed by David Tennant
; 2005 - 2010)Eleventh Doctor
(portrayed by Matt Smith
; 2010 - )