The Doctor (Peter Capaldi) merges from the TARDIS in a snowscape and falls to the ground, beginning to regenerate. In a flashback, the Doctor tests Missy (Michelle Gomez) by having her answer a distress call, with Bill (Pearl Mackie) and Nardole (Matt Lucas). Answering the call, they land on a giant colony ship trying to escape a black hole. A crewmember named Jorj (Oliver Lansley) holds them at gunpoint, demanding to know who is human. Bill admits to being human and he shoots her dead. Humanoid figures in medical masks and gowns take her away to restore her life. The Doctor, by telepathy, tells her to wait for him.

The Doctor, Missy, and Nardole are told by Jorj that some of the human bridge-crew had gone down to the lower level to start the engines and never returned. Jorj says the ship had 50 souls aboard two days ago, but Nardole can tell there are thousands of lifeforms below. Time moves faster in the lower level, farther from the black hole, and they are the crew’s descendants. They figure two years have already passed for Bill. They incapacitate Jorj and take the lift.

Bill wakes up in the hospital to find that she has an artificial heart. She is shown about by one of the crew, named Razor, who explains that some of the crew get upgraded for Operation Exodus for immunity to the pollution in the ship. Years later, they see the Doctor and the others coming down the lift, but she is tricked into an upgrade.

When they arrive, the Doctor and Nardole explore the hospital while Missy tries to determine the ship’s origin. It turns out to be Mondas, Earth’s twin. Razor tells her she has been here before and that the Doctor will never forgive her. He removes his disguise and is revealed as the Master (John Simm), Missy’s previous regeneration. The Doctor and Nardole find a Mondasian Cyberman, who claims to have been Bill Potts. The Cyberman chides the Doctor, saying, “I waited for you,” and sheds a tear.

These Cybermen are the same as those who appeared to the First Doctor in the episode The Tenth Planet in 1966, which means that this is a prequel to the first appearance of the Cybermen. The episode was loved overwhelmingly. It was called macabre and riveting. John Simm is a bearded Master like Roger Delgado and Anthony Ainley. Bill’s tear is reminiscent of that shed by Captain Picard when he is Borgified. The ship is 400 miles long and 100 miles wide. This is the first episode in which the Doctor himself says he is Doctor Who.