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.

Laurène comes face to face with a wolf which appears to be guarding a newborn baby hidden in a box. She undertakes to find the baby's parents, followed from afar by the mysterious animal. As everything rapidly becomes complicated, memories from her troubled past resurface.

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.