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 was in the Season 21 serial "Frontios"; Mark Eden appeared as the title character in the Season 1 serial "Marco Polo"; Nicholas Briggs, who has voiced the Daleks since the series was revived in 2005, portrayed his own predecessor; and Brian Cox voiced the Elder Ood in "The End of Time". Jean Marsh and Anneke Wills, who both played companions to Hartnell's First Doctor also appeared during Verity Lambert's leaving party scene. The drama was watched by 2.71 million viewers in the UK. Rotten Tomatoes gave it a 95% approval rating based on 22 reviews, and an average score of