The Doctor is time traveling adventurer of the great and old race known as the Time Lords. A common trait among his species is the "regeneration cycle", which means that a Time Lord can die and reincarnate 12 times before being effectively killed.
An illustration of the Doctor's ship, the TARDIS (which takes the form of a blue police box) can be seen in a map of the Blazing World.
The second incarnation of the Doctor appeared in 1969.
The First and Eleventh incarnations of the Doctor appear and run off, shortly before Andrew Norton, another time traveller, shows up.
The Doctor is from BBC's long running TV-series Doctor Who . Due to copyright reasons, the Doctor is never referenced by name.