Six months after Bill consented to the Monks taking over the world in the previous episode, the Monks have absolute control over everything, including thought -- to the point where everyone on Earth believes that the Monks have been in control for decades, rather than half a year.
The only one to know the truth of the matter appears to be Bill herself, who has been trying to find Nardole and the Doctor himself, who has been participating in the Monks' propaganda broadcasts.
Nardole actually locates Bill instead, revealing he's found the Doctor. The Doctor initially tricks Bill into thinking he's actually working for the Monks, but after she shoots him, he reveals it was only a test to see if she was under the Monks' control.
The Doctor and Bill consult Missy, who suggests killing or rendering Bill brain dead. The Doctor comes up with an alternate plan to override everything through Bill's psychic connection with the Monks.
Unfortunately, the Doctor's plan goes awry, and he is unable to complete the override. Bill decides to take his place. Her own memories start to get corrupted, until she remembers her mother and the love they shared.
Once people start rebelling, the Monks are sent packing pronto.