When the body of a young woman is found beneath the Aurora Bridge, forensic psychologist Daphne Matthews is first at the scene. The victim's name is Mary-Ann, her boyfriend has a record of violence, and her grieving brother is out for revenge. Detective Lou Boldt is investigating a series of mysterious disappearances, and both Boldt and Matthews are led into the 'Underground' - a perfectly preserved city-under-a-city, hidden beneath Seattle. Then the stalking begins: eerie phone calls, noises outside the house, shadows in the night. Someone has their eye on Matthews - but do they want to stop her, kill her, or help her solve the crime?