The Doctor (Jodie Whittaker) and her companions Yaz (Mandip Gill) and Dan (John Bishop) intervene in a Cybermaster attack on a bullet train in space, but Dan is nearly killed. He decides to leave the Doctor to get back to living his life. The Cybermasters teleport away with a child who was being held on the train. A renegade Dalek contacts the Doctor to give her info about a Dalek plot to destroy humanity. The Doctor tracks the child to a second moon that appears in the sky in 1916 Russia—really a planet converted by the Cybermen. The Doctor and Yaz find the child from the train imprisoned there and learn that it is actually an enslaved energy being called the Qurunx.
The Doctor’s former companions Tegan Jovanka (Janet Fielding) and Ace (Sophie Aldred) are investigating the abduction of a dozen leading seismologists and the strange addition of Grigori Rasputin’s face to fifteen famous paintings. Kate Stewart (Jemma Redgrave) of UNIT invites the Doctor to their headquarters to meet Tegan and Ace and discuss the case. The Doctor knows that Rasputin’s face is actually that of the Master (Sacha Dhawan) and she confirms this at a seismology conference in Naples. The Master has shrunken and killed all the seismologists.
The Master and his Cybermaster allies have joined with the Daleks to destroy the human race by triggering eruptions in every volcano on Earth. The Doctor meets with the renegade Dalek and learns that it was contacted to capture her. It is killed by the other Daleks. The Doctor’s ally Vinder (Jacob Anderson) crash lands on the Cyberplanet.
UNIT arrests the Master, who sent a miniature version of Ashad (Patrick O’Kane) to Tegan, who thought it a warning from the Doctor. It begins to enlarge and becomes like a Russian nesting doll, bringing Ashad and a bunch of Cybermen into UNIT headquarters to free the Master. The Daleks take the Doctor to the Master in 1916 Russia, where he is impersonating Rasputin. The Master uses Gallifreyan technology and the Qurunx to force regeneration, turning the Doctor into him. The Doctor’s image talks to images of the first, fifth, sixth, seventh, and eighth Doctors (David Bradley, Peter Davison, Sylvester McCoy, and Paul McCann), who advise her that the forced regeneration can be undone.
An AI program the Doctor created, using images of the fifth, seventh, eighth, and thirteenth doctors leads Ace to meet with former companion Graham O’Brien (Bradley Walsh) and destroy the Dalek volcano machine, Tegan to destroy the Cyberman converter and Cybermasters in UNIT headquarters, and Yaz and Vinder to capture the Master and make him undo the forced transformation. Though defeated and dying, the Master wounds the Doctor with the Qurunx’s energy beam, triggering her genuine regeneration.
The Doctor takes Yaz home and leaves alone. Yaz joins a support group of former companions, including Graham, Dan, Kate, Ace, and Tegan, Ian Chesterton (William Russell), Jo Jones (Katy Manning), and Melanie Bush (Bonnie Langford). The Doctor travels to a lonely cliff where she regenerates into the fourteenth Doctor, but actually it’s the tenth Doctor (David Tennant).
The episode was written by Chris Chibnall and directed by Jamie Magnus Stone. It was called an epic blockbuster special, and indeed it was, for the Centennial of the BBC. Tom Baker declined to appear. William Russell, who had last played Ian Chesterton in 1965, received a Guinness World Record for longest absence before returning to a role. David Tennant’s lines as the shocked tenth Doctor appears were written by himself: “What? What?” The episode garnered 100% on Rotten Tomatoes. Jodie Whittaker was pregnant at the time and rather ill but didn’t tell anyone. She may not have been the most memorable Doctor, but her energy was unsurpassed and she never lost the human touch.