A long time ago, the Doctor is sent to execute Missy, only to have Nardole interrupt on the behest of River Song. He refuses to kill Missy, instead opting to imprison and guard her for 1000 years.
In the present, the Pope comes to the Doctor, asking to help translate a text called “Veratis.” Everyone who has translated it has committed suicide, and the Doctor, Nardole, and Bill are brought to the Vatican to investigate. While there, Bill and Nardole discover a portal, which leads them to the Pentagon. They find the portals being projected leading all over the world.
The Doctor temporarily restores his sight using Time Lord technology, but is ambushed by aliens. Nardole realizes the projectors are not projecting portals, but the whole world. His realization causes him to dematerialize.
Bill finds the Doctor, who tells her the world is a simulation, and Veratis contains the proof. Bill is killed by the alien, the Doctor realizing this is a test to see if they can conquer the Earth. However, the simulation is too exact, as his sonic sunglasses still work perfectly, and the virtual Doctor sends his information to the real one to warn of the coming invasion.