Doctor Who is a British science fiction television programme broadcast by BBC One since 1963. The programme depicts the adventures of a Time Lord called "the Doctor", an extraterrestrial being who appears to be human. The Doctor explores the universe in a time-travelling space ship called the TARDIS. Its exterior appears as a blue British police box, which was a common sight in Britain in 1963 when the series first aired. With various companions, the Doctor combats foes, works to save civilisations and helps people in need.
Beginning with William Hartnell, thirteen actors have headlined the series as the Doctor, and in 2017 Jodie Whittaker became the first woman to play the role. The transition from one actor to another is written into the plot of the show with the concept of regeneration into a new incarnation, a plot device in which a Time Lord "transforms" into a new body when the current one is too badly harmed to heal normally. Each actor's portrayal is unique, but all represent stages in the life of the same character, and together, they form a single lifetime with a single narrative. The time-travelling feature of the plot means that different incarnations of the Doctor occasionally meet.