Suspected of abusing a prisoner during an interrogation, Major Rossi Vogel is interviewed by the IGS. Placed in custody, he takes the only path that seems open to him.

To avenge the insult done to their leader, buried without honors, Eddy, Theo, Walter and Roxane cross the yellow line. They remove Benaissa for questioning, very muscular, and manage to snatch the name of his accomplice, but accidentally kills Theo. Despite the release of their alibis, the four colleagues find themselves in the crosshairs of the IGS and the prosecutor. They are constantly questioned by Vogel and his men.

Eddy, Theo, Walter and Roxane are sinking more and more done in a spiral of violence and resentment. Eddy and Walter have two mobsters who killed Walter owed money. The homicide squad is dispatched to the offices of the Judicial Police and the General Inspection Service is on a war footing. Caught by Lemoine, Theo had to give the name of his accuser, who is none other than his lawyer. Roxane's worried about his colleagues, made of a cycle in which the young woman does not want to set foot.

After an explosive action, Eddy is hospitalized. The noose is tightening around the team after the formalization of the investigation by the IGS. The order comes from above: Vogel has carte blanche to bring down the group, provided conclusive evidence. Walter is kidnapped just outside his home. Her captors, all ready to torture, demanding a ransom of 200,000 euros and require that the case remains secret.

Eddy and Theo joined in on a robbery Lemoine. Their goal is simple: they hope to gather the ransom to secure the release of Walter. But they realize too late that the bandit has deceived. Indeed, the judicial police is already there. Narrowly, they manage to escape by taking hostage Roxane. For his part, Bordier was able to identify the kidnapper of Walter. This is a bloodthirsty mercenary named Zoran. Meanwhile, Vogel steps up efforts to monitor the team that is becoming more likely.

Roxane's father died of a heart attack after a threatening visit to Vogel. Meanwhile, Eddy and Theo found Lemoine. At the police station, a compromising video of the robbery could allow identification of Eddy and Theo. Bordier was able to locate the place where Walter Zoran holds. Joined by Eddy, Bordier and his band of former police intervene, thwarting the spinning of the IGS. The means used by Vogel to drop the team are increasingly sadistic.

Lemoine's lawyer is found dead. The prosecutor says the investigation Vanderbeke to the Central Office for the Repression of Banditry, led by Commander Marceau. Eddy and his team must find the first mobster to avoid being compromised. During this hunt, the death of Rossi takes a new light when his wife, Helen, documents discovered in a secret safe. While Walter and Theo's friend questioned Lemoine, Eddy goes to one of his contacts to find it. But Commander Marceau managed to find Lemoine together the team of Eddy.

Lemoine is in police custody, he immediately asked to speak only with Caplan. Eddy talks with him and promises to get him out, on pain of being denounced by Lemoine and go to jail for 20 years. Lemoine was summoned to the courthouse. Eddy and his three companions are an escape plan in motion, with the aim of killing Lemoine once escaped. The latter managed to escape from the courthouse. Lemoine They bring in a forest, and kill him. Once Lemoine knee, it shows them that he had rolled: he wore a microphone and was able to forward the discussions with Eddy and other cops, the prosecutor Vanderbeke. Eddy and Theo, who braquaient Lemoine, release their gun.

They've barely solved the Russian Mafia case when Caplan, Walter and Roxanne find themselves faced with "Baba" Aroudj, head of the Turkish Mafia. As for Vogel, he finds himself in a mess in prison. But that's not enough for Caplan, who wants to definitively put him out of harm's way. Only to get to him, Caplan is going to have to reconnect with the ghost of a very painful personal past.

The police of police isn't done with the Departmental Service of the Judicial Police (SDPJ) group. The investigation into Caplan's group and Bernardi is led by a devious cop: Chief Superintendent Henri Brabant. For his part, Walter is still enraged about the violent aggression his children suffered. Behind Caplan's back, he enlists Wassim Alcala, an old friend from the army, to find Charlie (Caplan's mysterious friend who appeared in season 3) to equip him which what he needs to take down the Turks.

The first commando operation that Caplan, Walter and Wassim lead against the Turks, puts Baba Aroudj under the fire within his organisation.

The great Manitou of the Turkish Mafia organisation has decided to let Baba Aroudj go. Fatih Imrali shall serve as an intermediary with Caplan and Walter.

By intervening in Marseilles to capture "Baba" Aroudj, Caplan and Walter interfere in an investigation into a bloody heist, committed ten years earlier in Aubagne. Commander Guido Frankeur, who never dropped the case, intends to charge Caplan and Walter for their interference. Unless the two Parisian cops help him to ruin the Turkish Mafia case in league with the Marseilles mobsters.

Caplan and Walter are struggling in the Marseilles bouillabaisse while in Paris, Roxanne and Bernardi find themselves in the middle of a drug deal that leads directly to mobsters Caplan knows well, having used their "services".

Caplan and Walter return to Paris. A return motivated by a blackmail forcing Caplan to organise the escape of a mobster who could ruin him by reporting him to Brabant, the Superintendent of the police determined to dismantle the SDPJ group and disgrace Caplan. At the same time, Georges Mandeville is in Paris to find some cash. Lots of cash!

It's the home stretch for Caplan, Walter and Roxanne. At the end of this path of extreme justice that they've carved themselves, with their noblest intentions, love, family and death await them. And they will all pay a heavy price.