Deep in the forest, the body of a young woman is found hanging from a tree. Major Laurène Weiss attempts to solve the mystery while also welcoming an eccentric new prosecutor to Villefranche. Together they investigate, in a town already under pressure from the recently announced closure of the local sawmill and the disappearance of the Mayor's daughter.

An amateur caver emerges traumatised from a labyrinth of subterranean grottos where he says he lost his friend. Laurène tries to unravel fact from fiction in his witness statement before contemplating a perilous descent. At the same time, Cora thinks that the Mayor's daughter has been kidnapped by a mysterious group of activists who are leaving their marks on the walls of Villefranche.