It picks up six months after the previous episode, when Bill handed control of Earth over to the Monks. Fear can lead people to give power to the strong willed, trading freedom for security. Once that power is given, it is much harder to take it back.
The Doctor has used that time to build a small resistance group, but that is not enough to win. He approaches Missy in the Vault à la Hannibal Lecter for information on how to beat them. She explains that since Bill made the agreement with the Monks, she is the psychic link and to break their hold on Earth, she must die.
This psychic link is used by the Monks to project themselves into the world’s memories. Humanity now believes that the Monks have been a part of our development from the beginning and were present throughout all our history. From a narrative standpoint, I find it interesting that the Monks are essentially the opposite of the Silence. The Silence had been shaping human development, but erased themselves from our memories; whereas, the Monks implanted themselves in our memories and took credit for our accomplishments.
When they reach the propaganda broadcast center, Bill saves the day by transmitting thoughts and images of her late mother (taken from the photos the Doctor gave her earlier this season) to replace the Monks in people’s memories. It is a somber reminder that love and compassion are the tools needed to destroy fascism.