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
William Hartnell "The Name of the Doctor". The following story and the 50th anniversary special of the show, "The Day of the Doctor", featured two new pieces of dialogue for Hartnell's Doctor, recorded by John Guilor, who had previously voiced the actor in a reconstruction of "Planet of Giants". Hartnell's Doctor also made a silent cameo appearance in the Series 9 episode "The Witch's Familiar" (2015) again played by a body double. David Bradley, who portrayed Hartnell in the biopic "An Adventure in Space and Time", played the First Doctor in the Series 10 episode "The Doctor Falls", and the 2017 Christmas