At the South Pole after an encounter with the Cybermen, the first Doctor refuses to undergo his regeneration. As he walks back to his TARDIS, he meets his twelfth incarnation after his own ordeal with the Cybermen and finds that he is in a similar state of mind. The two Doctors notice that the snowflakes have stopped falling, and are soon joined by an injured and confused man wearing the uniform of a British Captain from the First World War. All three of them, along with the first Doctor's TARDIS, are abducted by a spaceship.
On board the ship, they meet Bill, although the twelfth Doctor has doubts that she is real, despite her assertions to the contrary, having seen her converted into a Cyberman. They meet the pilot of the ship, a figure made of glass, which the Doctors observe is not symmetric. The pilot offers them freedom in exchange for the injured Captain, and reveals that he has been displaced from December 1914 while he and a German soldier were in a gun point stalemate. He only ended up at the South Pole because of an error during his displacement. The travellers refuse the pilot's offer and escape with the TARDIS, with which they navigate to the planet of Villengard, located at the centre of the universe.
After the two Doctors dodge some laser fire coming from a tower, the twelfth Doctor enters it and finds a rogue Dalek named Rusty, who had taken refuge from the other Daleks hunting it inside the tower. Rusty gives the Doctor access to the Dalek Hivemind, and there he finds out that the pilot and its ship, called the Testimony, were both created on New Earth, and their function is to extract people from their timelines at the moment of their death and archive their memories into glass avatars. The first and twelfth Doctors both find out that Bill Potts is actually a Testimony avatar that has been created from her memories.
Having found nothing evil to fight, the two Doctors agree to return the Captain to his proper timeline. When they do so, the Captain asks them to keep an eye on his family, and introduces himself as Archibald Hamish Lethbridge-Stewart (his descendant, Alistair Gordon Lethbridge-Stewart, would become a close friend of the Doctor's during the time he holds the rank of Brigadier). The snow begins to fall again as time resumes, and Captain Archibald finds himself once again in a gun point stalemate with a German soldier. Moments later, the Doctors watch the German soldiers sing a verse of "Stille Nacht", to which the British soldiers reply with a verse of their version of the carol, "Silent Night". As they sing, both sides lay their weapons down and meet each other under a flag of truce. The Twelfth Doctor explains to the first that he had returned Archibald to a point two hours ahead of where the Testimony had first taken him from so that he could experience the Christmas truce, which made sure that his life would be spared.
With Archibald's life saved, the first Doctor informs his future counterpart that he is now ready to regenerate, and says goodbye to him before returning to his own TARDIS. The shot of David Bradley alone in the console room dissolves into the scene with the first doctor, and we see the regeneration into his second incarnation. With only Bill's avatar for company, the twelfth incarnation still insists that she is not the real Bill Potts, but she answers that memories are what define a person. In order to help the Doctor understand, the Testimony send him avatars of two of his former companions, Clara and Nardole, and restore the Doctor's memories of Clara that he lost. The Doctor refuses to give a testimony of his life to the avatars, and the Testimony respect the Doctor's wish to be alone. The avatars of Clara and Nardole leave after embracing the Doctor.
The Doctor returns to his TARDIS and leaves some advice for his future self before he regenerates into his thirteenth incarnation, who looks into a mirror to find out what she looks like now. The TARDIS's cloister bell begins to sound as a result of it suffering multiple failures due to damage sustained during the regeneration process. As she loses control, the Doctor tumbles out of the TARDIS, and sees the console room exploding before the ship dematerialises while she plummets toward Earth.
William Hartnell (archive)
Tom Baker (af; uncredited)
Peter Davison (af; uncredited)
Chris Eccleston (af; uncredited)
John Hurt (af; uncredited)
Sylvester McCoy (af; uncredited)
Paul McGann (af; uncredited)
Jon Pertwee (af; uncredited)
Matt Smith (af; uncredited)
David Tennant (af; uncredited)
Patrick Troughton (af; uncredited)
|Mark Gatiss||The Captain|
|Nikki Amuka-Bird||Helen Clay|
Osian Andrew (uncredited)
David Cromarty (uncredited)
Aaron Hooper (uncredited)
Lewis Reynolds (uncredited)
|Nicholas Briggs (voice)||Daleks|
|Anneke Wills (archive)||Polly|
|Michael Craze (archive)||Ben|
|Mickey Lewis (uncredited)|
Daniel Woolf (uncredited)
- This special was the fifth and final Doctor Who Christmas special to feature the Twelfth Doctor. The previous four were "The Time of the Doctor", "Last Christmas", "The Husbands of River Song" and "The Return of Doctor Mysterio".
- This special was the first Doctor Who Christmas special to feature The Thirteenth Doctor. It was also the first episode to feature the new Doctor.
- Due to William Hartnall's death in 1975, David Bradley was cast as The First Doctor. Bradley had previously portrayed Hartnall in An Adventure in Space and Time, which was a one-off drama about Hartnell's time playing the Doctor. This programme aired as part of Doctor Who's 50th anniversary in 2013.
|Doctor Who Christmas specials|
|Tenth Doctor||"The Christmas Invasion" • "The Runaway Bride" • "Voyage of the Damned" • "The Next Doctor" • "The End of Time"|
|Eleventh Doctor||"A Christmas Carol" • "The Doctor, the Widow and the Wardrobe" • "The Snowmen" • "The Time of the Doctor"|
|Twelfth Doctor||"Last Christmas" • "The Husbands of River Song" • "The Return of Doctor Mysterio" • "Twice Upon a Time"|
|See also||"The Feast of Steven" • "Attack of the Graske"|