Peter Capaldi (Twelfth Doctor)
David Bradley (First Doctor)
Jodie Whittaker (Thirteenth Doctor)
Pearl Mackie (Bill Potts)
Mark Gatiss – The Captain
Jenna Coleman – Clara
Matt Lucas – Nardole
Nikki Amuka-Bird – Helen Clay
Toby Whithouse – German Soldier
Lily Travers – Polly
Jared Garfield – Ben Jackson
Nicholas Briggs – Voice of the Daleks
Directed by Rachel Talalay
Written by Steven Moffat
Produced by Peter Bennett
Incidental music composer
Length: 60 minutes
Originally broadcast 25 December 2017
Wandering back to his TARDIS at the South Pole, the First Doctor, refusing to undergo regeneration and leaving his companions behind, encounters the Twelfth Doctor outside his own TARDIS, in a similar state of mind after his previous ordeal. The pair are soon approached by a confused and injured First World War British captain, displaced from December 1914 while in a gun-point stalemate with a German soldier. All three are then abducted by a large spaceship, where 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 the moment of his death. Refusing to allow the Captain to die, the party escapes and takes the First Doctor's TARDIS to the planet Villengard.
Alone, the Twelfth Doctor meets with the rogue Dalek Rusty, who has taken refuge from the other Daleks hunting it. Given access to the Dalek Hivemind, the Doctor learns that the pilot and its ship, known as Testimony, were created on New Earth, designed to extract people from their timelines at the moment of their death, and archive their memories into glass avatars. "Bill" is one such avatar, created from her memories. Seeing no evil to fight, the Doctors agree to return the Captain to his timeline. Upon doing so, the Captain asks the Doctors to keep an eye on his family, introducing himself as Archibald Hamish Lethbridge-Stewart, an ancestor of the Doctor's close friend the Brigadier. As time resumes, the Doctors watch as soldiers on both sides begin singing "Silent Night". The Twelfth Doctor explains to the First that he deliberately shifted the Captain's timeline to the start of the Christmas truce, to ensure his life would be spared.
With the Captain saved, the First Doctor informs the Twelfth that he is prepared to regenerate and bids him good-bye before returning to his TARDIS. Now alone with Bill's avatar, the Doctor adamantly contends she is not the genuine Bill, only for her avatar to explain that memories are what define a person. To help him understand, the avatar restores the Doctor's memories of Clara Oswald before they are joined by Nardole's avatar. The Doctor refuses to give the avatars testimony of his life. They respect his wish to be alone and leave after he embraces them both. The Doctor then returns to the TARDIS and decides to regenerate, but not before relaying advice to his next incarnation.
While the Thirteenth Doctor examines her reflection, the TARDIS suffers multiple failures. Tumbling out of the ship, the Doctor observes the console room explode before the TARDIS dematerialises, as she plummets towards the Earth below.