All that was known about him in the programme's early days was that he was an eccentric alien traveller of great intelligence who battled injustice while exploring Time and Space in an unreliable old time machine called the
TARDIS. (The TARDIS is much larger on the inside than on the outside, and is stuck in the shape of a 1950s-style British
police box.)
The initially irascible and slightly sinister Doctor quickly mellowed into a more compassionate figure and it was eventually revealed that he had been "on the run" from his own people, the
Time Lords of the planet
Gallifrey. Also, like all Time Lords, the Doctor had the ability to "
regenerate" his body when near death.
A Time Lord can regenerate twelve times and the Doctor has gone through this process (and its resulting after-effects) on nine occasions, with each of his incarnations having his own quirks and abilities
