On an enormous colony ship trying to escape from a black hole, the Doctor (Peter Capaldi) discovers that Bill (Pearl Mackie) has been converted to a Cyberman. The Master (John Simm) and his current incarnation Missy (Michelle Gomez) capture the Doctor, but he has programmed the Cybernet to target Time Lords as well as humans, and they have to flee. Nardole (Matt Lucas) has commandeered a shuttlecraft for their escape. The Doctor is electrocuted by a Cyberman, but Bill saves him.
They escape to a higher level of the ship, where a solar farm harbours children and their adult guardians battle primitive Cybermen. The Doctor recovers, but he is fighting off regeneration. Bill does not fully understand that she is a Cyberman until a child reveals the truth. Missy and the Master discover a camouflaged elevator which may provide an escape route. But when they call for it, a Cyberman arrives, which they destroy. The Doctor warns them that the Cybermen dwell on the level where time moves more swiftly, so they have lots of time to evolve.
Nardole discovers fuel pipes below the farm. He triggers explosions which make the humans seem more powerfully armed. The Doctor orders Nardole to take the farmers to another farm on a safer level. Bill stays with the Doctor. The Master and Missy abandon them and take the lift to the lowest level to escape in the Master’s TARDIS. Missy betrays the Master and stabs him, but he shoots her in the back with his laser so she can’t regenerate. They laugh over their mutual betrayal and the Master leaves.
An army of Cybermen arrives and the Doctor is shot. Surrounded, he ignites all the fuel in the pipes and the Cybermen are destroyed. Bill kneels beside him. Suddenly, she is restored to humanity by her ex, Heather, who has followed her tears. She transforms Bill into a space-creature like herself. They leave the Doctor inside his TARDIS and Bill departs to explore the universe with Heather. The Doctor awakes, still fighting his regeneration, and hears the cloister bell as he meets with his first incarnation.
The First Doctor is played by David Bradley, who played William Hartnell in An Adventure in Space and Time. This is the first time two incarnations of the Master appeared simultaneously--both Michelle Gomez and John Simm. The episode was loved. The Doctor and the bickering Masters were singled out for praise. It was nominated for a BAFTA Award. The Tenth Planet episode in 1966 was not only the first appearance of the Cybermen, but it also featured the first Time Lord regeneration. Three different generations of Cybermen appear in the story.