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
The Chase (Doctor Who) the last appearance of Russell and Hill as companions Ian and Barbara and the introduction of Peter Purves as new companion Steven Taylor. In the TARDIS, the Doctor, Ian, Barbara, and Vicki are huddling around the Time-Space Visualiser, a television-like souvenir from their recent adventure at the Space Museum, which can pick up on any past event in the whole of time and space. The TARDIS then lands, and the Doctor confirms conditions are hospitable. Ian and Vicki leave to explore the desert wilderness. They find an ancient trap door in the sand. Inside, they are trapped — and become