Can MI5 prevent a nuclear disaster?

When Harry learns that an Iranian arms courier in London is carrying a circuit board giving Iran instant nuclear powers, he orders all planes grounded. But with Ros's bugs all over the Grid, Harry's plan is leaked and MI5 is warned that London's water supply will be poisoned unless the courier can enter Tehran. As Harry orders a security sweep on the Grid, Ros desperately tries to get rid of the bugs. Gaining intelligence that the courier is British, the team soon establish which plane he's on and Adam and Ros both board undercover – Adam as a passenger and Ros as an air stewardess. As the plane draws nearer to Iranian airspace, Adam accidentally kills a man he's convinced must be the courier, but then pinpoints the pilot as the culprit. But when Adam is arrested after his deadly assault, it's up to Ros to apprehend the pilot - and prevent a worldwide catastrophe. VIDEO: Watch a preview of Spooks