From the planet of Gallifrey comes a mysterious alien only known as "the Doctor". The show began with the idea of an educational program focusing on history but it ended up being the longest science fiction tv show in history.
The Doctor travels in a Time/Space machine called the TARDIS, an acronym that stands for Time And Relative Dimensions In Space; the machine would normall appear like a plain smooth box but wit hthe use of it's chamaleon circuit to blend into it's surroundings where it lands (anywhere in time or space) the Doctor's TARDIS is stuck like a 60's Police Box.
Through his travels he's battled Daleks, Cybermen, Autons and the Nestene Consciousness, a planet of Spiders, historical figures, Slitheen and even a megalomaniac computer who took on his likeness during his 4th life. Hundreds of baddies make up the list he's gone up against.
There have been 12 Doctors....the genius idea of the Doctor dying and regenerating enabled he show to be on so long. The 12th Doctor was played by Peter Capaldi. The 13th Doctor is played by Jodie Whittaker.

The original run was from 1963 to 1989 then it returned for 2005 with a bigger budget and a new actor, Christopher Eccleston, as the 9th incarnation of the Doctor.
Along his travels he picks up friends, "companions" as fans call them; they act as the in between to ask the questions so the viewer understand what sort of technobabble the Doctor's going on about.