During a handover to the head of counter-terrorism of MI5, Harry Pearce, a terrorist escapes custody. When Harry disappears soon after, his protégé is tasked with finding out what happened as an impending attack on London looms, and eventually uncovers a deadly conspiracy.

Defending their country, deceiving their friends — it's all in a day's work for the agents at MI5, the Security Service. The elite team tackle organised crime, espionage and terrorist activities, as well as the conflicts and power struggles back in the office. There's a car bomb in Liverpool, a siege in the Turkish embassy and the murder of a double agent, not to mention the difficulty of having to lie to loved ones about who you are and what you do.