On Christmas Day, in Paris, Place de Bitche, a hand cut out with a saw and clinging to a tree is found by Commissioner Rousseau's team. To solve this dark investigation, his hierarchy imposes on him the help of an atypical profiler, Mariella De Luca. Despite his strong reluctance, Rousseau accepts this partnership and seems convinced that this hand is the third murder committed by a serial killer.

A head is found in the catacombs of Paris. Commissioner Rousseau and his team quickly find the identity of the victim. This is Maximilien Müller, a renowned artist. But where is the body? The investigation leads the police to the painter's studio where they find not only the naked body of the artist but also that of a woman in her sixties, also decapitated. A head is found later but it cannot belong to the body found in the apartment. Is he a serial killer? Are the victims related?

A teenager, Anna, is found on the banks of the Seine, with her throat slit. Commissioner Rousseau, haunted by the demons of the past, abandons Commander Mariella De Luca, who finds herself alone facing this sensitive investigation and the mother of the teenager, Katia. Rousseau has decided to resume alone the investigation into the disappearance of his son, who left five years ago for a trip from which he never returned. In an autumnal Paris, characters rub shoulders, haunted by their respective quests, while the murderer continues to kill. Will Rousseau manage to find a second wind to lead his team again and restore Mariella's confidence?

In front of a Parisian pastry shop, a young student collapses, shot dead. No murder weapon, no suspect, nothing in the victim's past to follow any leads. A perfect murder with no motive. A new investigation by Inspector Rousseau of Commander Mariella de Luca.