A local figure is found murdered on the banks of the Ardèche river by young kayakers, in the town of Vallon-Pont-d'Arc. The murder weapon is a carved stone dating back more than 40,000 years, a period corresponding to the presence of men in the rock cave of the Region. Gendarmerie captain Riad Lekcir, in charge of the investigation, must team up with his counterpart in the research section, Manon Ferret-Duval. A collaboration that does not delight him at all because a painful past binds them. In December 1994, parallel to the discovery of this cave, Kamel, the older brother of Riad, was arrested, accused of having killed the parents of Manon (who was 8 years old at the time). Kamel committed suicide in police custody, seeing his alibi collapse. Our investigators will have to put aside this heavy past in order to carry out this investigation which will finally establish a link between these two murders perpetrated 30 years apart.