A murder case rocks a small town when Julien Monnier is found dead, stabbed near the cave dwellings known as "Les grottes du Régulus." An old book is found beside him. Grace, a police captain, is put in charge of the investigation and discovers similarities with a ten-year-old case: a 17-year-old girl, Sarah, had been found dead, also stabbed, in the same caves. Grâce calls on Alex, her childhood ...