The Doctor decides to put Missy's apparent desire for redemption to the test by dispatching her, Bill, and Nardole to respond to a distress call from a colony ship.
The only remaining crewman, a blue alien named Jorj, reveals that monsters took the others, all humans, away below decks, and none of the crew were ever seen again.
He shoots Bill to prevent the creatures from taking any of them, but they show up anyway and take her away below decks. She revives in a hospital with a new cybernetic chest plate keeping her alive.
She meets a rather off-kilter person named Razor, who explains that all the people below decks are dying because of the poor living conditions, and they need to be improved in the hospital in order to survive.
It's also revealed that since the ship is near a black hole, there's a time differential between the lower decks, where Bill is, and the upper decks. Time is passing much more quickly for Bill.
The Doctor, Nardole, and Missy board a lift to go to the lower decks to find and rescue Bill. Bill goes to meet them, but Razor instead leads her to a conversion theater.
Missy accesses the computers and discovers that the colony ship was from Mondas. Razor meets up with her and reveals that she was already on this ship before -- he's actually the Master (John Simm), her past incarnation.
The Doctor and Nardole discover the conversion theater and meet a Mondasian Cyberman. The Doctor begs it to tell him where Bill is, only for the Cyberman to reveal that it is Bill.
The Master and Missy both appear and reveal that this is the Genesis of the Cybermen.