An Iranian arms courier in London has procured a circuit board that will finally enable Iran's nuclear warheads. In order to prevent the boards reaching Tehran, Harry orders all planes grounded.
But with Ros' bugs in place on the Grid, word of this plan leaks fast and MI5 receives a threat that a lethal chemical will be released into London's water supply unless the courier is able to reach Tehran. Harry orders a security sweep on the Grid as Ros desperately attempts to dispose of the bugs.
Armed with further intelligence that the courier is British, the team manage to establish which plane he is on. Adam and Ros both board undercover - Adam as a passenger and Ros an air stewardess.
Meanwhile, Jo and Malcolm find the source of the poisoned water supply and must find a way to prevent the release of the chemical, without the terrorists learning that their threat has been foiled.
As the plane draws nearer to Iranian airspace, tension mounts - Adam and Ros must identify the courier from the passenger list, or ground the plan, before it enters Iran. Adam accidentally kills a man from the CIA he's convinced is the courier. He's quickly detained by Iranian police and soon discovers the pilot's the culprit.
Adam and Ros are too late in acquiring the circuit boards that will give Iran instant nuclear capability.