The main character is known only as 'The Doctor'. In a very early episode, Barbara Wright, a teacher who is destined later to become one of the Doctor's companions refers to "The Doctor" and Ian Chesterfield, her boyfriend and also a teacher about to take a trip or two in space and time in the TARDIS asks, "Doctor who?" Hence the series title. They meet the Doctor after following his niece, Susan, (who they are both intrigued by and who is a pupil at their school), home in the fog to a junk-yard where the Tardis is concealed.
The Doctor is not subject to the normal constraints of mortal life as he is a Time Lord, a race from the planet Gallifrey that has mastered the secrets of time, but which for the most part keep them secrets. His first incarnation was played by the irascible William Hartnell, and early in the series viewers were indoctrinated in the mysteries of his TARDIS, a machine capable of travel through both space and time. It was not long before The Doctor and his (often fallguy) companions fetched up against his ultimate enemies, the Daleks, a lethal race of metal-armoured mutants, whose chief role in the great scheme of things would appear to be, as they frequently observe in their instantly-recognisable metallic voices, to 'Exterminate!'. Having successfully defeated the Daleks on a number of occasions, The Doctor has so far kept the universe safe from their depredations.
The Doctor has also saved the Earth (and a number of other planets) from such notable adversaries as the Cybermen, the Sontarans and the Silurians. His whereabouts are currently unknown, although a later reincarnation, Sylvester McCoy is rumoured to be about to rematerialise in the forthcoming series, Death Comes To Time.
Eight actors played the Doctor in the original series: William Hartnell (1963-1966), Patrick Troughton (1966-1969), Jon Pertwee (1970-1974), Tom Baker (1975-1981), Peter Davison (1981-1984), Colin Baker (1984-1986), Sylvester McCoy (1987-1996) and Paul McGann (1996-). The changing of actors is explained within the series by the Time Lords' ability to "regenerate" after suffering mortal injury, illness, or age; the process repairs and rejuvenates all damage, but as a side-effect it reconfigures the Time Lords' appearance semi-randomly and often has effects on their personalities. This explanation was not developed until after the elderly William Hartnell had already retired from the show for health reasons.
The Doctor was played in the film versions (Doctor Who And The Daleks in 1965 and Daleks -- Invasion Earth 2150 AD in 1966, both essentially retellings of existing episodes on the big screen and with a bigger budget) by the actor Peter Cushing.
Doctor Who has appeared on stage, numerous times. Novelisations of the TV series exist, as do a number of series of original novels, some of which are well-regarded and officially continue the show's canon. The show was renowned for its use of innovative music and special effects which were produced by the BBC's Radiophonic Workshop. The pilot episode for a potential spinoff series was aired in 1981, K-9 And Company: A Girl's Best Friend by Terence Dudley, but was not picked up as a regular series.
Efforts continue to revive Doctor Who, either on television or as a feature film; only time will tell if any of these are successful. However, "official" (which is to say, BBC-sanctioned) Doctor Who survives in a number of forms. BBC Books produces a series of original novels, with two books are published each month except December; one features the ongoing adventures of the Eighth Doctor and the other an "untold" story of an earlier Doctor. Big Finish Productions makes a range of audio plays on compact disc, with one released every month starring one of the Fifth, Sixth, Seventh and Eighth Doctors. Finally, there is a regular Eighth Doctor comic strip in the pages of Doctor Who Magazine, published four-weekly by Marvel UK. A new quarterly line of Doctor Who novellas is also en route from Telos Publishing.