Members of an al-Qaeda cell abduct a British soldier home on leave from a tour in Iraq, and threaten to execute him unless the Government cancels the country's Remembrance Sunday commemorations. MI5 secure the release of Lucas North, an MI5 agent captured by the Russian FSB eight years before. Arkady Kachimov, head of the FSB's London operation, offers to exchange intelligence on a Muslim terrorist network in exchange for support for an Anglo-Russian energy deal. Fearing that her cover has been compromised, Harry recalls Ros Myers from assignment in Moscow.

Harry is determined to take revenge on Arkady Kachimov for his complicity in the attempted Remembrance Sunday outrage. Ros coordinates an attempt to expose FSB plans to destabilise the UK. Lucas could be a valuable MI5 asset, but Harry is uncertain where his loyalties lie after eight years in Russia.

Ben infiltrates an al-Qaeda cell and uncovers a plan to mount a bombing campaign in London. Lucas reveals that the Russians are aware of Operation Sugarhorse, suggesting that the FSB have a highly-placed mole within MI5. Harry seeks the assistance of his mentor, the Spycatcher.

A senior al-Qaeda operative offers to provide intelligence on a planned terror attack in exchange for the public pardon of two prisoners. The FSB order Lucas to ensure that the bomb goes off.

Ros and Lucas go undercover to investigate the role of a prominent businessman in the destablisation of the British banking system.

Section D uncovers an MI6 plot to assassinate a U.N. negotiator, in London to attend an emergency peace summit following an Israeli assault on Gaza. Lucas tries to discover the identity of someone selling elements of an assassin's toolkit. Harry receives information about the highly-placed mole from a sleeper in Russia.

Operation Sugarhorse is compromised and Harry is arrested on suspicion of being the Russian mole within MI5. Lucas travels to Russia to obtain intelligence that may secure the UK against the threat of nuclear attack. Ben investigates irregularities in the circumstances leading to the passing of an asset.

Section D uncover details of Tiresias, a network of Russian sleepers tasked with crippling the UK's infrastructure ahead of a nuclear attack. They turn to the traitor Connie James for assistance, but the FSB are determined to recover their asset.