The First Doctor

Wandering back to his TARDIS at the South Pole, after defeating the Cybermen (see "The Tenth Planet"), the First Doctor realises that he is about to regenerate. He refuses to do so and leaving his companions, Ben Jackson and Polly Wright, behind he encounters the Twelfth Doctor outside his own TARDIS, in a similar state of mind after his previous ordeal with the Cybermen (see "World Enough and Time/The Doctor Falls"). The Twelfth Doctor is unable to convince The First Doctor that they are the same person when time suddenly stops causing the falling snow to become static, mid-air.

Meanwhile, in 1914 a soldier, a captain in the British Army, lies in a shell hole on the battlefield in Ypres. His revolver is aimed at a German soldier, who also has his sidearm trained on the Captain. Both men realise they will kill each other in self-defence, their respective troops far away in their trenches, unable to assist either of them. Just as the Captain prepares to accept his fate and fire his gun at the German soldier, a wave of energy flows through the battlefield, freezing everything around him. As the Captain exits the hole, a bright light suddenly flashes and encapsulates him. He then finds himself in a snowy landscape and, on hearing voices, he approaches the two people who are having a heated discussion about the static snow.

The First Doctor and the Twelfth Doctor become curious about the Captain’s appearance but soon become distracted by a portal of light that has opened in the distance. As the two Doctors approach it the First Doctor orders the portal to reveal itself. However, the light quickly vanishes, prompting the First Doctor to tell the Captain to enter his TARDIS, and the Twelfth Doctor coaxes him to follow. As they enter, the First Doctor is shocked by the changes to the TARDIS interior, while the Captain is shocked by the room's size. The Twelfth Doctor attempts to convince his younger self they are the same person by displaying that he too is undergoing regeneration. He also points out that the First Doctor's TARDIS is 70 feet away, and they are inside a later version of the TARDIS, hence the differences in appearance. Regardless, the First Doctor remains sceptical. The First Doctor notices the Captain's unease, telling the soldier to sit down and orders the Twelfth Doctor to fetch some brandy.

Ben and Polly Help The First Doctor

Suddenly, the TARDIS shakes violently, prompting the Twelfth Doctor to attempt to take off, only to find that the engines are unresponsive. Looking upwards, out of the TARDIS doors, he sees large metal claws pulling the TARDIS upwards. Once the TARDIS has stopped moving the Twelfth Doctor persuades his predecessor to confront their captors. On exiting the TARDIS the First Doctor finds himself in a huge spaceship. A humanoid glass figure seated at the top of a staircase speaks to the First Doctor, addressing him as ‘The Doctor of War’. The figure requests the return of the Captain in exchange for a friend of The Doctor.

Much to the confusion of the First Doctor, the Twelfth Doctor’s former companion Bill Potts then appears from one of the side corridors. The Twelfth Doctor quickly exits the TARDIS to hug her, before slowly pulling away, drawing his sonic screwdriver, and scanning Bill. To her surprise, the Twelfth Doctor demands that she proves herself to be real, as he believes Bill to have sacrificed herself to defeat the Cybermen. Bill insists she is her real self, explaining how Heather had saved her. The Twelfth Doctor asks where Heather is now. Bill however, struggles to give an answer.

The Twelfth Meets The First

The Twelfth Doctor decides to confront their captors directly and runs up the staircase. He is slowly followed by his younger self, who questions the need to move so fast. They find an empty chair with several panels behind it. The First Doctor takes to observing the panels, commenting on their extra-terrestrial origin. He turns to his older self, his eye-glass dropping from his eye as he notices the Twelfth Doctor wearing sunglasses to scan the panels. They turn to the chair, where a woman made completely of glass materialises. While the Twelfth Doctor scans the panels with his sonic screwdriver, the First Doctor looks more closely. He berates the Twelfth Doctor for not noticing the asymmetrical features of the woman's face, blaming his use of the sunglasses instead of his eyes. The Twelfth Doctor shyly agrees, and his younger self throws the sunglasses to the ground.

The Twelfth Doctor presses on with his plan to escape, using the sonic screwdriver to activate the crane which pulled the TARDIS up to take them back down to the ground, the team jumping onto the chains to escape as well. Their captors however, refuse to let them escape, halting the chains and pulling the TARDIS back into the spaceship. However, both incarnations of The Doctor, Bill and the Captain manage to jump to the snowy ground before the TARDIS is pulled back up.

Bill expresses concern about what they will do without the TARDIS but both incarnations of The Doctor reveal they have another, younger version, at their disposal. They head to the First Doctor's TARDIS, swiftly entering. As she does so, Bill comments on how much smaller it is, causing the Twelfth Doctor to briefly observe the windows before insisting his younger self to take off ‘deep space, anywhere’.

The Captain

While in flight the Twelfth Doctor attempts to access the databanks but quickly realises they are rather scarce, given the TARDIS is still young. To compensate, he sets the TARDIS to land at the centre of the universe, Villengard, impressing his younger self at how easily he is able to navigate the TARDIS.

As soon as the TARDIS materialises they start to search the area. The Captain however, is attacked by a creature with multiple little green legs but they save him, both Doctors recognising the creature as a Kaled mutant. The Twelfth Doctor insists Bill wait in the First Doctor's TARDIS to look after the shocked Captain. The First Doctor fetches the Captain another brandy before joining his older self outside. However, while Bill looks after the Captain she grabs the brandy and her hand is revealed to be glass.

Meanwhile outside the two Doctors rest, discussing their respective regenerations. The Twelfth Doctor remarks they have yet to change because they are in a ‘state of grace’, though he himself finds his body starting to deteriorate. He asks why his younger self halted the regenerative process, unable to remember himself. The First Doctor admits he is afraid about regenerating. Before the Twelfth Doctor can answer the question himself they suddenly come under attack from a creature in a nearby tower. The attack only stops when the Twelfth Doctor asks the creature to scan him, announcing ‘I'm dying!’. He then goes into the tower, telling his younger self to wait outside to which the First Doctor begrudgingly agrees.

The Captain and Bill Potts

While waiting the First Doctor observes the area, finding the remains of a Dalek casing. As he inspects it Bill sneaks up on him and begins asking why he ran away. The First Doctor insists she return to the Captain but Bill persists by asking him what he was running to. Interested by the question, The First Doctor responds that he left to discover why good always defeats evil. Bill hugs him, thanking him for his efforts. However, when she pulls away, the First Doctor realises that Bill is a glass avatar.

Meanwhile inside the tower the Twelfth Doctor meets with a rogue Dalek called Rusty, who has taken refuge from the other Daleks who are hunting it. He remarks on how he is a 'Good Dalek' though Rusty continues to fire at the Twelfth Doctor. The Twelfth Doctor decides to come to a truce to which Rusty agrees by disarming his gunstick. Given access to the Dalek Hivemind, the Twelfth Doctor learns that the pilot and its spaceship, known as Testimony, and were created on New Earth (see "New Earth" and "Gridlock"). It is 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. Time suddenly freezes again as the First Doctor and Bill enter the room. She explains she is there to convince The Doctor to return the Captain back to his timeline. Disappointed at seeing no evil to fight, both incarnations of The Doctor agree to take the Captain back to the World War One battlefield.

Upon doing so, the Captain asks the Doctors to keep an eye on his family, introducing himself as Archibald Hamish Lethbridge-Stewart. The Twelfth Doctor instantly realises that the Captain must be an ancestor of his close friend Brigadier Alistair Lethbridge-Stewart (aka The Brigadier). The First Doctor assures he will do so, the Twelfth Doctor confirming they did. The Testimony reveal that the Captain will not remember the incident and a perception filter will ensure that both incarnations of The Doctor are not noticed once time restarts. As time resumes they watch as soldiers on both sides start to sing Silent Night. Soon, each side disarms their weapons and enter the battlefield, leading the Captain and the German soldier to withdraw their weapons and make peace. The Twelfth Doctor explains to the First Doctor that he deliberately shifted the Captain's timeline up a few hours, to the moment of the Christmas truce in 1914 where the soldiers stopped shooting and came together for Christmas, to ensure his life would be spared.

Helen Clay

With the Captain saved, the First Doctor informs the Twelfth Doctor that he is prepared to regenerate and bids him good-bye before returning to his own TARDIS where he regenerates into the Second Doctor. As the soldiers play football and spend Christmas Day together, the Twelfth Doctor watches on, saluting the Captain - who briefly bypasses the perception filter, but dismisses him and returns to his trench. Now alone with Bill's avatar, the Twelfth 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 Twelfth Doctor's memories of Clara Oswald, bringing him joy, before they are joined by Nardole's avatar.

The Twelfth Doctor however, refuses to give the avatars testimony of his life, stating he must be left alone before he changes. They respect his wish and leave after he embraces the avatars of his former companions. The Doctor then returns to his own TARDIS and, after arguing with his time-machine, decides to regenerate, but not before relaying advice to his next incarnation. He collapses as he does so but manages to return to his feet 'One more lifetime won't kill anyone, except me'. With that, he regenerates, igniting the console room as his face and body changes.

Smoke fills the console room, the Twelfth Doctor's ring falling to the floor. As the smoke clears, the new Thirteenth Doctor, now a female, examines her reflection, before saying 'Oh, brilliant'. She then presses a button before the TARDIS suffers multiple failures. Tumbling out of the time-machine, the Thirteenth Doctor observes the console room exploding. She then watches in horror as the TARDIS dematerialises while she falls through the air to the ground below.