New revelations about how agents of British intelligence infiltrated the Irish Republican Army. By 1979, the British security forces believed the IRA had become so security conscious that they were impossible to penetrate. But reporter Jennifer O’Leary reveals how one weakness in the IRA’s internal security was exploited to unlock many of the group’s secrets. She charts how Britain used informers and combined that advantage in secret intelligence with the use of special forces to take on one of the IRA’s deadliest units – a strategy that culminated with the Loughgall ambush, when the SAS killed eight IRA men attacking a police station. The programme shows that the aftermath of the attack only made the IRA’s informer problem worse.