On an unforgiving winter morning, a group of workers discover the headless body of a horse, hanging suspended from the edge of a frozen cliff. Commandant Martin Servaz starts investigating and soon discovers a dark story of madness and revenge.
After finding a hair of Hirtmann bathed in the blood of the head of the decapitated horse, Servaz collapses. What if Hirtmann manipulated them from the bottom of his cell? At the Warnier center, Diane confronts Hirtmann for the first time - which reveals her true identity and the real reason for her presence at the center. The investigation of Servaz and Ziegler is directed towards a certain Garaudy, notable of the region. Servaz, in spite of his reluctance, will in turn confront Hirtmann - who predicts that a guilty man will die the same evening.
Perrot, a close friend of Garaudy, is also found dead. Again Servaz and Ziegler arrived too late. They plunge into the suicide case, and discover that Garaudy and Perrot were very close to the current mayor of Saint-Martin, Chaperon. Will Servaz and Ziegler avoid new murders?
Chaperon, the mayor of Saint-Martin, was killed without being able to say anything - but the killer escaped Servaz. Soon, Servaz and Ziegler discover unsustainable videos featuring Chaperon, Perrot and Garaudy, as well as two other predators tracking teenagers as wild beasts. Who are the two unknowns? Who could have decided to take revenge 15 years later? For her part, Diane learns of Servaz's responsibility in the death of her sister.