The protagonist of the series is the Doctor, a Time Lord from the planet Gallifrey. He travels through time and space battling evil in a time machine called a TARDIS ("Time And Relative Dimensions In Space"), which looks like a London police box - a sort of bobby's phone booth - but is bigger on the inside than the outside.
The Doctor's name is not Who (that's the name of the series), and in fact is not known at all.
Being a Time Lord, the Doctor has the fantastic ability to change his form and his whole manner every time an actor tires of the part. Seven actors played the Doctor during DOCTOR WHO's 26-year run on the BBC, and the last of those appeared in the 1996 movie just long enough to "regenerate" into the eighth.
In March 1999 a spoof of DOCTOR WHO with a "ninth" incarnation appeared in a BBC world children's hunger telethon. In November 2003 an animated DOCTOR WHO adventure appeared on the BBC's website featuring the "offical" ninth incarnation. Both of these incarnations appear in my fanfiction.
The Doctor almost always travels with one or more companions; "companion" has achieved the status of technical term among DOCTOR WHO creators and fans. Most usually the companion is an attractive young girl, but since DOCTOR WHO began as a family series - and arguably always has been - the series has little or no overt sexual innuendo, and fan fiction little more.
The Doctor has had several recurring adversaries during his tv run. Among them are the Daleks, mutants who live totally enclosed in single-passenger mobile travel devices, believing themselves to be the supreme beings of the universe and destined to rule it and "Exterminate!" all others. The Daleks were first featured in the second DOCTOR WHO story, and became an overnight sensation, setting DOCTOR WHO on its course as an English tradition for more than a quarter of a century.
Another recurring adversary is the Time Lord known as the Master. He's forsaken the society of Gallifrey to travel the universe like the Doctor, but instead of battling evil is out to conquer the universe. His rivalry with the Doctor goes back to what the Doctor refers to as their school days, and his plot to take over the universe rarely fails to provide for a humiliating death for the Doctor ... and, in fact, rarely fails to fail.
DOCTOR WHO impresses me as the most versatile format for screen adventure stories ever created. With the TARDIS the Doctor can go anywhere, be any time, do anything. Given a regeneration or two he can be anyone (even, as in Trial of a Time Lord, his own villain).
Science fiction author Harlan Ellison is among the legions of fans of DOCTOR WHO, for its unadorned straightforward storytelling style. So far DOCTOR WHO has failed to gain wide exposure to American audiences, but I'm confident that it will; and when it does, it will finally take the place it deserves with the major adventure folklore of this age, such as STAR TREK, STAR WARS, James Bond, and Superman.
For a more practical if tongue-in-cheek demonstration of what DOCTOR WHO is, see The Unearthly Enemy Child Within of Terror, a sort of pilot satire episode.
*****
Except that its protagonist is female, and she's an adolescent instead of a milleniarian, and she's an Earth high school student instead of a Time Lord, and she's the Chosen One rather than a self-appointed hero, and has a Hellmouth in a small town as her base of operations rather than traveling the universe going to look for trouble, and she has a Watcher and her high school friends to tell her things instead of knowing everything herself, and that it's horror instead of science fantasy, BUFFY THE VAMPIRE SLAYER is exactly like DOCTOR WHO.