While investigating a young man’s apparent suicide in a Helsinki subway station, Detective Sergeant Timo Harjunpää of the Helsinki Violent Crimes Unit is baffled by the apparent lack of witnesses or evidence. Even the closed-circuit security tapes offer no clues. As a pattern of death emerges, it becomes apparent that there is a serial killer at work—a deluded and dangerous fanatic who inhabits an underground bunker under a desolate Helsinki hillside. No stranger to the seamier side of human nature, Detective Sergeant Harjunpää must now apprehend the most terrifying adversary of his career.