The TARDIS materialises in a pitch-dark tunnel, where the Doctor, Amy and Rory stumble on the dead body of a soldier. Questioned by his superior officer, Colonel Bowe, they learn that they're inside a British nuclear bunker, in the middle of an atomic war — in 1982.

Amy and Rory weren't even born then, but they know the bomb didn't drop that year, and so does the Doctor. The friends also know they had nothing to do with the death of Sergeant Trott — so who, or what, was the killer? And why does the Doctor's psychic paper not work on the Colonel?

The Doctor, Amy and Rory soon learn that something else is lurking in the shadows. Something deadly...