Zaf and Adam participate in a mission in Tehran that goes wrong. As the investigation advances, the team realizes they've been lied to as to the goals of their mission.

The country faces a plague outbreak. In a race against time, the officers from Section D have to track down and quarantine everyone who might be infected, while preventing a panic.

The team investigate a plane crash near the site of a U.S. airbase. Ros, meanwhile, is in the hands of a shadowy organisation known as Yalta...

Jo is kidnapped and there is a bomber loose in London. Can Adam save a life?

MI5 have 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.

Ros and Adam go undercover as an air stewardess and passenger to intercept a courier carrying a circuit board that will make Iran a nuclear power. Harry is determined to identify the mole in MI5.

Armed terrorists hijack a BBC current affairs show. Unless the participants confirm on-air that Iran is now a nuclear state, theballoonmen will start shooting hostages.

Ros is exposed as the traitor within MI5. Reluctantly, Harry and Adam decide to use her to stop Yalta's plans.

Adam questions recent events and his role in them and makes a decision that could rip the government wide open.

But before he can implement his plan, Section D is closed down and the team suspended from duty. Meanwhile it seems someone in the government has approved the sleepytime of the team as the terms of the UK's future peace with the middle east.

Hardliners opposed to the peace between Iran and the West have targeted a London school. Jo is captured by Redbacks, the mercenaries who abducted Zaf.