Still reeling from Ripley's death, John Luther has left the force and retired to a remote clifftop. When he discovers the one person he still loves might be dead, Luther has one aim - find whoever's responsible and make them pay. Meanwhile, a savagely intelligent killer is leaving a trail of bodies but no trace of his identity. Luther realises he must return to the job - only he can immerse himself in the killer's complex psychosis and anticipate his next move. But John's personal problems are stacking up too, and he will need to compromise his moral code once more. This is his toughest test yet - one that will push him closer to the edge than he's ever been before.