Tensions are high in 1972 London when the Soviets devise Operation Glass, a plot to bring down Britain, while the world's superpowers engage in a war fought mainly by spies. A defecting KGB officer contacts MI5 with details of the plan. In response, MI5's leader assembles a secret team, including enigmatic agent Joe Lambe, to investigate and stop the threat to Britain and the rest of the world. Among its tasks is to locate enemy sleeper agents across the U.K. before they can carry out their roles in the conspiracy.