When a six year old girl is found dead, hanging from a tree, the only clue the Oslo Police have to work with is an airline tag around her neck. It reads 'I'm travelling alone . Holger Munch, veteran police investigator, is immediately charged with re-assembling his homicide unit. But to complete the team, he must convince his erstwhile partner, Mia Kruger a brilliant but troubled investigator - to return from the solitary island where she has retreated with plans to take her own life. Reviewing the evidence, Mia identifies something no one else has noticed a thin line carved into the dead girl's fingernail: the number 1. Instinctively, she knows that this is only the beginning. To save other children from the same fate, she must find a way to cast aside her own demons and confront the most terrifying, cold-hearted serial killer of her career...