During the restoration works in the castle of Annecy-le-Vieux, the workers hear screams coming from the building and see a man running in flames and falling through the castle walls. He is the mayor of the town, Christian Verneuil, whose charred corpse is found by the PJ at the foot of the castle, with a broken neck and a strong smell of oil. Evidences in the building show that it is murder. Enzo Mattei, one of the volunteer workers, has a record and is the main suspect.

Anicée Fromentin finds her parents at home shot dead, Julien Fromentin, worker for the French Forestry Agency, and Lise Blandin, psychiatrist. Louis Grandet, a neighbour, responds to Anicée screams and calls the police. Jean-Paul recognizes him as an old police colleague. Grandet remembers hearing two shots he thought were from hunters. In the morning Lise had found the word "murderer" painted on their garage door that she wanted to erase before Julien found it.

An attractive woman appears dead wrapped in a para-glider, what at first looks like a sports accident will quickly turn into murder. Commissioner Cassandre will have friction with the prosecutor, who is the mother of her captain.

Manon Favre, a 22-year-old student, is found dying of an overdose on a hiking trail. All the evidence leads to a drug trafficker who wants revenge on the father of the girl, a volunteer in a neighborhood association that fights drugs in the street.

At Annecy Palace of Justice, Procureur Roche makes the final statement in a trial, when a man from the public benches stands up and claims for the justice he was denied. As the court agents try to reduce him, he threatens to activate the detonator of the explosives belt with 3 kilos of ammonium nitrate he is carrying and asks the Procureur to call his son, Capitaine Roche. He is Paul Sorbier, a surgeon who worked for 30 years in war zones with the Red Cross and who demands that the case of the alleged suicide of his daughter, Marina Joussineau, be reopened. He blames Roche for a judicial error and demands that her son investigates to repair it.

Major Antoine Richeux is shot in the forehead while hunting a wolf, Cassandre and his team must investigate the division, bringing to light various cases of sexual abuse.

A young man is found murdered in his home, with an unconscious man lying next to him and the murder weapon in his hand. When he wakes up in the hospital, he not only remembers nothing, but barely claims to know the victim at the sawmill co-op where they both work and has no apparent reason to do so.

At the shelter "La première pierre", Adam Martinet gives the last instructions to the volunteers who prepare the annual open day. Meanwhile, he looks for Chloé Legrand, but finds her killed by a blow to the back of the neck. Chloé, 35, single and childless, was the director of the center. According to Nathalie Rouget, veteran volunteer, Chloé had run away from a husband who mistreated her. The judicial PJ finds signs of struggle in the victim's house.

Vannier, a colleague of Philippe Cassandre, ex-husband of Florence, is found dead in a contraband truck. Investigation reveals that he had two daughters whom he abandoned 20 years ago and recently joined and could profit from his death.