The Tardis arrives on a large 400 mile long space that is on the edge of a black hole but is being held back. When the occupants including this Missy excluding The Doctor leave the Tardis, they are confronted by a blue skinned alien where a confrontation results in Bill Potts being shot and then being taken away by medical personnel who arrive at the control room.
Throughout the episode, Bill Potts is seen walking walking around a large hospital where there are other people with white cloths over their heads. Bill befriends a person who looks like Rasputin, the mad monk.
Due to time and gravity, the time in the hospital travels much faster than the time in the control room. Whilst its minutes for The Doctor, its days and weeks for Bill Potts. The floor that she is on is a virtual reality room where its not only the hospital and surrounding buildings. The floor is more like a whole city and Bill is able to visit outside the hospital when her friend is able to take her out.
The Doctor encounters the Rasputin caretaker who reveals himself to be The Master. The Doctor is not able to downgrade Bill Potts to human form.
The Master takes a captured Doctor to the top of the building where the Master plans to escape with Missy using the spaceship that Nardole had managed to obtain. The Doctor had managed to reprogramme the ships computers earlier and had made the Timelords targets of the Cybermen which had prompted the Master to escape.