In the Basque region of southern France, in the area of Saint Jean de Luz, a young woman is found dead after being tortured, the victim of an ancient religious ritual frequently performed in the area during the Middle Ages. Those methods were used at the time for the women that were convicted of being witches. Marie Daguerre (38 years old), a rookie detective newly promoted in the district of Saint Jean de Luz, is asked to lead the investigation on her first murder case. Meanwhile in Paris, Vincent Becker (50 years), a mythical figure in the police force, receives a distressed call for help from his son Mathias, whom he hasn't heard from in years, and who is in deep trouble. The call came from a small town in the Saint Jean de Luz area. Now, on top of her own case, Marie is forced by her superiors to help Vincent Becker track down his son. It soon becomes clear the two cases are linked, and, as more victims turn up, that Mathias Becker is somehow involved.