The programme depicts the adventures of a mysterious and eccentric humanoid alien known as the Doctor who travels through time and space in his spacecraft, the TARDIS (an acronym for Time And Relative Dimension In Space), which normally appears from the exterior to be a blue 1950s British police box. With his companions, he explores time and space, faces a variety of foes and rights wrongs.
The Doctor has been played by eleven actors. The transition from one actor to another is written into the plot of the show as regeneration, whereby the character of the Doctor takes on a new body and, to some extent, new personality. Although each portrayal is different, and on occasion the various incarnations have even encountered one another, they are all meant to be aspects of the same character. The Doctor is currently portrayed by Matt Smith, who took up the role after David Tennant's final appearance in an episode broadcast on 1 January 2010.
The programme was intended to be educational and for family viewing on the early Saturday evening schedule. Initially, it alternated stories set in the past, which taught younger audience members about history, with stories set either in the future or in outer space to teach them about science. This was also reflected in the Doctor's original companions, one of whom was a science teacher and another a history teacher.
Doctor Who is one of the five top grossing titles for BBC Worldwide, the BBC's commercial arm. BBC Worldwide CEO John Smith has said that Doctor Who is one of a small number of "Superbrands" which Worldwide will promote heavily.