Doctor: Peter Capaldi (Twelfth Doctor)
Pearl Mackie (Bill Potts)
Matt Lucas (Nardole)
Michelle Gomez – Missy
John Simm – The Master
Oliver Lansley – Jorj
Paul Brightwell – Surgeon
Alison Lintott – Nurse
Nicholas Briggs – Voice of the Cybermen
Directed by Rachel Talalay
Written by Steven Moffat
Produced by Peter Bennett
Executive producer(s) Steven Moffat
Incidental music composer Murray Gold
Series Series 10
Length 1st of 2-part story, 45 minutes
Originally broadcast 24 June 2017
In the cold open, the Doctor emerges from the TARDIS onto a snowscape and collapses, beginning to regenerate.
Earlier, the Doctor proposed to test Missy's atonement by having her, alongside Bill and Nardole, answer a distress call. They arrive via the TARDIS on a giant colony ship reversing away from a black hole. They are held at gunpoint by the solitary pilot, Jorj, who demands to know which of them is human, fearing creatures that are arriving by the lift. Bill admits to being human and Jorj shoots her. Humanoid figures, wearing cloth masks and hospital gowns, take Bill's body away claiming they can restore her. Right before the lifts depart, the Doctor telepathically tells Bill to wait for him.
The Doctor, Missy, and Nardole learn from Jorj that, two days previously, some of the human bridge crew had gone down to the lower level of the ship to start the engine reversal, but they never returned. Jorj claims the ship only had 50 people two days ago, but Nardole shows there are actually thousands of life forms below. They are the descendants of the crew: due to time dilation from the black hole, time moves faster at the ship's lower levels than the bridge, and a year has already passed for Bill. The Doctor incapacitates Jorj and, along with Missy and Nardole, departs below in a lift.
Bill awakens in a hospital, where she finds that she has been fitted with a mechanical device that serves as her replacement heart. Razor, one of the hospital's employees, shows her around and explains that some of the patients are waiting to be "upgraded" for "Operation Exodus", to escape the polluted air of the ship's lower levels. Years later, they see footage from the bridge of the Doctor coming down the lift. However, Razor then tricks Bill into becoming the next "upgrade" subject.
Upon arriving, the Doctor and Nardole explore the hospital while Missy tries to determine the origin of the colony ship. She finds the vessel is from a "twin" of Earth, Mondas. Razor approaches her and insists she has been here before, and that the Doctor will never forgive her for what has happened to Bill. When she denies it, Razor removes his disguise, revealing himself to be the Master, Missy's earlier incarnation. The Doctor and Nardole find an operating theatre where a "Mondasian" Cyberman, from when the Doctor first encountered them, emerges from a closet. The Cyberman identifies itself as formerly being Bill Potts. The Master and Missy then explain they are all witnessing the genesis of the Cybermen. The Cyberman tells the Doctor "I waited for you"; underneath its face cover, Bill sheds a tear.