A young man enters the culverted remains of an ancient Glasgow stream, looking for thrills. Deep below the city, the tunnels are decaying and claustrophobic, more so with every step. As the ceiling lowers to no more than a couple of feet above the ground, the man makes a gruesome discovery. DI Rachel Narey leads the official investigation while photographer Tony Winter follows a lead of his own through the shadowy world of urbexers, people who pursue a dangerous and secretive hobby, a world that Winter knows more about than he lets on. It soon becomes clear that the murderer has killed before, and has no qualms about doing so again.