Doctor: Peter Capaldi (Twelfth Doctor)
Pearl Mackie (Bill Potts)
Matt Lucas (Nardole)
Michelle Gomez – Missy
John Simm – The Master
Samantha Spiro – Hazran
Briana Shann – Alit
Rosie Boore – Gazron
Simon Coombs – Rexhill
Stephanie Hyam – Heather
Nicholas Briggs – Voice of the Cybermen
David Bradley – The Doctor
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 2nd of 2-part story, 60 minutes
Originally broadcast 1 July 2017
Aboard the colony ship reversing from the event horizon of a black hole, the Doctor finds Bill has been converted into a Cyberman. The Master and Missy capture him, but the Doctor had earlier surreptitiously reprogrammed the Cybernet to target Time Lords as well, forcing them to flee. Nardole arrives in a commandeered shuttlecraft to rescue them. The Doctor is electrocuted by a Cyberman, but is saved by Bill.
They evacuate to a higher level of the ship containing a solar farm populated by children and a few adults fighting off early Cybermen prototypes. The Doctor recovers, but suppresses the early signs of regeneration. Bill initially remains unaware of her transformation, her strong mind acting like a perception filter, until one of the children inadvertently reveals the truth to her. Bill sheds a tear, which the Doctor calls a hopeful sign. Missy and the Master discover a camouflaged lift, a possible escape route. But when they call for it, an upgraded Cyberman arrives, which the group destroys. The Doctor warns that the time dilation affords the Cybermen more time to evolve and strategize.
Nardole discovers the floor directly below the solar farm contains fuel pipes. He is able to trigger controlled explosions, which are used to exaggerate the humans' defensive strength and defeat the Cybermen's initial attack. Knowing this ploy only delays their inevitable defeat, the Doctor instructs Nardole to lead the human community to a solar farm on another floor and remain there to safeguard them. Bill stays to fight with the Doctor. Despite the Doctor's impassioned plea, Missy and the Master abandon them, intending to take the lift to the lowest level and escape in the Master's TARDIS. Missy, however, changes her mind and stabs the Master, which will trigger his next regeneration. He retaliates by shooting her in the back with his laser screwdriver, telling her she will not regenerate. Both laugh over the irony of their mutual betrayal before the Master departs.
An army of Cybermen arrive, but are warded off by the Doctor until he falls after being shot. Surrounded, the Doctor ignites all the fuel pipes, engulfing the farm in a fireball and destroying or disabling the Cybermen. The Doctor lies still as Bill kneels beside him. She suddenly finds herself outside of her Cyberman body in her human form. Heather ("The Pilot") appears, having found Bill through her tears. She saves Bill by transforming her into an entity like herself. They leave the Doctor inside his TARDIS, and Heather invites Bill to explore the universe with her. Bill sheds tears for the Doctor before leaving with Heather.
The TARDIS arrives in a snowy landscape and the Doctor awakens, briefly dazed and confused. Refusing to keep continually changing, he emerges and seemingly stops his regeneration. Within, the cloister bell sounds an alarm as the Doctor encounters his original incarnation.