The Doctor (Doctor Who) Time Lords a fixed number of twelve regenerations, meaning that every Time Lord had a total of thirteen incarnations including the original. The plot of "The Time of the Doctor" involves the Doctor receiving a new cycle of regenerations from the Time Lords before his expected demise, triggering the regeneration into the Twelfth Doctor, played by Peter Capaldi. The origins of the programme were explored in the docudrama "An Adventure in Space and Time" as part of the 50th anniversary celebrations of "Doctor Who", which starred David Bradley as William Hartnell. Although Time Lords resemble humans, their physiology differs in
An Adventure in Space and Time of 1960s Dalek props crossing the bridge, in a recreation of a famous scene from "The Dalek Invasion of Earth" (1964). Interior scenes replicating early "Doctor Who" production at Lime Grove Studios were also filmed, with replicas of early television equipment being used. A number of the cast have appeared in "Doctor Who" at one time or another, most notably original companions William Russell and Carole Ann Ford. David Bradley appeared in the Series 7 episode "Dinosaurs on a Spaceship", while Jessica Raine was in the Series 7 episode "Hide" and the "Call the Midwife" charity cross-over mini-episode; Jeff Rawle