"Twice Upon a Time" is an episode of the British science fiction television series Doctor Who, written by Steven Moffat, directed by Rachel Talalay, and was broadcast as the thirteenth (and to date final) Christmas special on 25 December 2017 on BBC One. It features the final regular appearance of Peter Capaldi as the Twelfth Doctor, the first official appearance of Jodie Whittaker as the Thirteenth Doctor, and guest-stars David Bradley as the First Doctor, having previously portrayed original First Doctor actor William Hartnell in the 2013 docudrama An Adventure in Space and Time. Pearl Mackie guest stars as the Twelfth Doctor's former companion Bill Potts, while his other companions make guest appearances – Jenna Coleman as Clara Oswald and Matt Lucas as Nardole. This episode is a continuation of events after "The Doctor Falls", addressing the cliffhanger that it ended on, and takes place during the final serial of the First Doctor, The Tenth Planet (1966), which ended with the introduction of the concept of a Doctor regenerating; footage from The Tenth Planet is used in the special. "Twice Upon a Time" is Capaldi's fourth and final Christmas special as the Twelfth Doctor, and the last Doctor Who story to be written and produced by Moffat, who had served as the show's executive producer and chief writer since taking over from Russell T Davies in 2010. After the special's broadcast, Moffat was succeeded as executive producer and showrunner by Chris Chibnall. It is also the last episode to have music composed by Murray Gold, who had composed music since the series' revival in 2005 and was succeeded by Segun Akinola in Series 11. The special has received generally positive reviews from critics. Plot Synopsis Wandering back to his TARDIS through the South Pole in 1986 after leaving his companions behind, the First Doctor refuses to regenerate. He encounters the Twelfth Doctor outside his own TARDIS in a similar state of mind. The pair are soon approached by a confused and injured First World War British captain (Mark Gatiss), displaced from December 1914 while in a gun-point stalemate with a German soldier. All three are then forcibly taken into a large spaceship. Inside, they meet with Bill Potts. The Twelfth Doctor, however, doubts she is the real Bill. Upon encountering the ship's glass-like holographic pilot, they are offered freedom in exchange for allowing the ship to return the Captain to