Trapped by Cybermen, Clara Oswald (Jenna Coleman) pretends to be the Doctor. Cybermen controlled by Missy (Michelle Gomez) rise into the air and explode over 91 areas of the U.K., creating clouds of Cyber pollen that resurrect millions of dead as Cybermen. One of these is Danny Pink (Samuel Anderson), the late boyfriend of Clara. UNIT takes Missy and the Doctor (Peter Capaldi) into custody on a UNIT plane, where the Doctor is inducted as President of Earth. Danny has kept his personality, even as a Cyberman, and saves Clara.

Missy escapes and kills Osgood (Ingrid Oliver), a UNIT scientist. A horde of Cybermen attack the plane. Clara uses the TARDIS phone to call the Doctor. Missy points out that she gave that number to Clara, then blasts open the cargo doors. Chief Scientific Officer of UNIT Kate Stewart (Jemma Redgrave) plummets to Earth as Missy teleports to safety. The Doctor takes the TARDIS to the graveyard. Danny reveals that a second rainfall will convert living humans into Cybermen.

Missy offers the Doctor a chance to control the Cyberman army, but he rejects that gift and passes the control bracelet to Danny, who sends the Cybermen into the sky, where they explode, stopping the rainfall. Missy claims that Gallifrey is back in its original location. Clara tries to kill Missy, but the Doctor stops her from being corrupted by revenge. The late Brigadier Lethbridge-Stewart, brought to life, if that’s the word, as a Cyberman, vaporises Missy. He has saved his daughter Kate.

Danny contacts Clara from the Nethersphere, a data-cloud storing dead minds. He has a chance to bring one person back to life before the Nethersphere shuts down, and he chooses the boy he accidentally killed as a soldier, asking Clara to return him to his parents. When Clara meets the Doctor later, he thinks that Danny has returned from the dead and lies about returning to Gallifrey, and she lies that she and Danny will be fine. In a mid-credit scene, Santa Claus (Nick Frost) knocks on the TARDIS door and tells the Doctor the truth.

Reviews of this episode were highly positive, particularly regarding Michelle Gomez as Missy, though the Cybermen were criticized. When Clara claims to be the Doctor in the cold opening, Coleman’s eyes replaced those of Peter Capaldi in the opening credits. There are several references to Missy’s past as the Master. Gallifrey is located in the Constellation of Kasterborous, Galactic Coordinatess 10-0-11-00:02.

The Doctor’s four wives are mentioned: Susan’s grandmother, Queen Elizabeth I, Marilyn Monroe, and River Song. Missy sings Happy Birthday to the Doctor, imitating Marilyn. Missy descends from the clouds with an umbrella like Mary Poppins, on whom the character was based. Doctor 12 likes lots of sugar in his tea, like Doctor 3 and Doctor 8. Matt Smith has a cameo as Doctor 11. The scene of unmasking Danny as a Cyberman was influenced by the unmasking of Darth Vader in Return of the Jedi.