An undercover MI5 agent has gone missing. It's up to Julian Mercer and his team to perform the recovery. There's just one problem. They don't know if the agent has been compromised or if he switched sides.

Agent Owen Shepherd was tasked with infiltrating an IRA terrorist faction. He's been in deep cover for two years, but now that an attack is imminent, he's vanished. While searching for Shepherd, Mercer stumbles upon the terrorist's plans. Mercer's new primary objective – stop the attack. His secondary objective – find Owen Shepherd.

In order to save lives, Mercer will have to step into Shepherd's shoes and get close to the faction leader. But the longer Mercer remains embedded with the terrorist cell, the deeper his distrust of the Security Service becomes, from the operation runner's loyalties to the real reason Owen Shepherd went missing. Did the terrorists silence Shepherd or did his own people turn against him?