The 1st Doctor (David Bradley as actor William Hartnell) wanders back to his TARDIS through Antarctica in 1986. He refuses to regenerate and encounters the 12th Doctor (Peter Capaldi), who also does not wish to regenerate. They meet an injured and somewhat shell-shocked First World War British Captain (Mark Gatiss), who has been misplaced from December 1914 during a stand-off with a German soldier (Toby Whithouse). All of them are taken into a large starship, where they find the 12th Doctor’s former companion Bill Potts (Pearl Mackie), though the Doctor himself is unsure she is real. The pilot of the spaceship is a holograph who seems to be made of glass (Nikki Amuka-Bird), who offers freedom to them all if they return the Captain to the moment of his death. They refuse, and they escape to take the 1st Doctor’s TARDIS to the planet Villengard in the far future.
The 12th Doctor runs into the rogue Dalek Rusty (Nicholas Briggs), who is hiding from the other Daleks. The Doctor obtains access to the Dalek hivemind and learns that the alien captain and its ship Testimony were created on New Earth to extract people from their timeline at the moment of their death and preserve their memories in glass avatars. Bill Potts was one such. The Doctors see no real evil afoot and agree to return the Captain to his death in his rightful timeline. Arriving back in World War One, he asks the Doctors to watch over his family and tells them his name is Archibald Hamish Lethbridge-Stewart, which means he is the ancestor of the Doctor’s long-time friend The Brigadier. Time resumes and the Doctors watch the famous Christmas Truce, when both sides of the war sang Silent Night together. The 12th Doctor admits that he monkeyed with the timeline somewhat to save Lethbridge-Stewart.
The 1st Doctor Now wishes to regenerate, says goodbye, and enters his TARDIS. He returns to the South Pole for his companions Ben (Jared Garfield) and Polly (Lilly Travers), and regenerates into the 2nd Doctor. The 12th Doctor, still with Bill’s avatar, still insists she is not really Bill, but she says she is Bill’s memory and that’s what we really are. She restores the Doctor’s memory of Clara Oswald (Jenna Coleman), and they are joined by Nardole’s avatar (Matt Lucas). The Doctor embraces them but they agree to leave him alone. He returns to his own TARDIS and resigns himself to being regenerated, after leaving a bit of advice to his next incarnation.
The regeneration is spectacular. The 13th Doctor is female (Jodie Whittaker), which is a bit of a shock to the Doctor. As soon as she touches the console, the TARDIS experiences multiple systems failures because of the delayed regeneration. The TARDIS dematerializes and the new Doctor plunges toward Earth.
The episode was written by Steven Moffat and directed by Rachel Talalay, presented as the Christmas Special of 2017 on BBC One. David Bradley had played the 1st Doctor actor William Hartnell in the 2013 docudrama An Adventure in Space and Time and has played the 1st Doctor off and on ever since. This was the last Doctor Who story written by Moffat and the last to feature music from Murray Gold. It was well-received by critics and received 88% positive on Rotten Tomatoes. It was nominated for a Saturn and was a finalist for a Hugo Award.
The episode featured clips from the 3rd, 5th, 8th, 9th, and 10th Doctors, and several others are referred to. Some of the clips used came from lost episodes but the clips themselves had survived. A trailer was shown at the 2017 San Diego ComicCon. The 1st Doctor as presented was criticized for being sexist, but the authors thought that this was not only accurate, but it made for interesting contrast with the 12th Doctor. David Bradley at one point speaks slowly as if trying to remember his lines, as William Hartnell sometimes did. Director Talalay did not know who the next Doctor would be and had to come back to direct the regeneration scene.