A British plane is thought to have been hijacked and taken to an American airstrip. Ros is captured by the Yalta organisation when the team shows up and they learn of an international cover-up.

MI5 has to intercept a Russian arms dealer who plans to pass blueprints for a nuclear device to Iran. Adam and Ana decide to bring their relationship to a close.

Harry discovers there's a mole at large in the Grid. Meanwhile, Ros and Adam board a plane undercover in an attempt to stop Iran becoming a nuclear power.

Ros is in grave danger after Bob Hogan discovers she's a traitor. Meanwhile, Harry's brought before Yalta's chief, whose identity shocks him to the core.