The Doctor (Peter Capaldi) is still blind from exposure to vacuum, but his sonic sunglasses give him a bit of sight, and no-one knows. He is asked by the Pope (Joseph Long) to investigate a recently translated text called the Veritas. People have committed suicide after reading it. With Bill (Pearl Mackie) and Nardole (Matt Lucas), he visits a secret library of banned works in the Vatican. Bill and Nardole find a portal leading to more portals, including the Nuclear Research Centre at CERN and the White House. Temporarily recovering his sight with Time Lord tech, he reads Veritas, but is interrupted by skeletal alien figures called Monks.

Bill and Nardole enter CERN and find the scientists preparing to kill themselves. When asked to pick a random number, they all pick the same one, and are freaked out. The time-travellers return to the portal hub, but Nardole discovers that the portals are only computer projections. Bill follows a trail of blood and finds herself in the Oval Office, where the Doctor is waiting. He explains that Veritas describes a demonic invasion using computer simulations of reality. He, Bill, Nardole and everybody else involved are simulacra in a simulation. Bill disappears like Nardole did. The Doctor simulacrum is helping prepare the invasion by sending everything through his sonic glasses to his real self.

The real Doctor finishes Extremis and goes to the vault under the university. Inside the vault is Missy (Michelle Gomez), whom he had saved from execution by the Time Lords by vowing to keep her locked up for a thousand years. He asks for her help in fighting the invasion.

The episode received excellent reviews for its unusual and complex script, fast pace, and sharp wit. Missy, as usual, steals every scene she is in. A musical motif from the James Bond movies is heard several times. What is unusual is that the Doctor never leaves Earth but appears throughout most of the story in a virtual simulation forwarded to the real Doctor by his sunglasses. Nardole compares the simulated world to the holodecks of Star Trek. This episode is the first of the Monks Trilogy, to be followed by The Pyramid at the End of the World and the Lie of the Land.