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.
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".