When a dangerous terrorist escapes on his watch, MI-5 agent Harry Pearce is held to blame, which causes him to vanish himself. When his protégé Will is brought in to aid in his capture, Harry ends up recruiting Will to find the real traitor inside MI-5 who aided the terrorist's escape.
Following the escape of a terrorist from an M15 agent who also disappears soon after, the agent's protégée sets out to find what happened as a new terrorist attack starts to haunt London.