Good dope is becoming rare in the North of Paris. Drugstores are being raided by junkies and gangs are nervous, fighting each others: the 'Viets', the 'Blackies', the 'Arabs', the neo-nazis 'Justiciers' and some mean gays. Vincent (Daniel Auteuil) is the good cop coming from Marseilles where he was a gangster. He's a soft method guy but also kicks assses hard and throw lethal dialog lines when needed. With the precious help of 'l'Arbalète'* (Marisa Berenson**), a tox' prostitute ex-member of Vincent's former gang, he will try to put order in that mess. There's also a violent and racist cop (Marcel Bozzuffi), Algeria veteran with hard methods, whose role could be more than to protect and to serve.

After leaving boxing, Stéphane is employed as a bodyguard for a politician. One night, Stéphane and his friend Pierre-Marie confront some belligerent billposters. Pierre-Marie is murdered. For Stéphane, the anger is so strong that he pursues the murderer, who dies accidentally. Accused of the crime, Stéphane seeks help from a journalist, who will help him get his side of the story heard.

Four murders in forty-eight hours happen in Nice. The murderer calls himself "The Exterminator" and the police are unable to catch him. Tom Lepski is an insurance detective and he get involved in the investigation when Liza Mendoza, a famous violinist and a friend, is killed just a few minutes before Tom visit her. After another rich woman killing, Tom finds out that all the victims were members of the exclusive Fifty Club. All of them but Lisa.

Having developed a revolutionary device that puts water in the mouth of all secret services in the world, professor Forrester is about to go on a conference in San Francisco under the protection of Tom Lepski, an insurer.