On the banks of the Ardèche, near Vallon-Pont-d'Arc, kayakers find the body of the museum president of a recently discovered Paleolithic cave. A 40,000-year-old carved stone was embedded in his neck. To investigate, Captain Manon Ferret-Duval of the research section teams up with her counterpart Riad Lekcir. They must overcome a shared past: she is convinced that her partner's older brother is responsible for the murder of her parents in 1994. Riad has no doubt that Kamel, now deceased, is innocent.