The Doctor is the protagonist of the long-running BBC science fiction television series Doctor Who. They are an extraterrestrial Time Lord who travels the universe in a time travelling spaceship, called the TARDIS, often with companions. Since the show's inception in 1963, the character has been portrayed by fourteen lead actors. The transition to each succeeding actor is explained within the show's narrative through the plot device of regeneration, a biological function of Time Lords that allows a change of cellular structure and appearance with recovery following a mortal injury.

The TARDIS is a fictional hybrid of a time machine and spacecraft that appears in the British science fiction television series Doctor Who and its various spin-offs. While a TARDIS is capable of disguising itself, the exterior appearance of the Doctor's TARDIS typically mimics a police box, an obsolete type of telephone kiosk that was once commonly seen on streets in Britain in the 1950s. Paradoxically, its interior is shown as being much larger than its exterior, commonly described as being "bigger on the inside".