In Paris, bloodthirsty jewel thief Roger Sartet (Alain Delon) escapes from custody with the help of the Manalese, a small-time but well-organised Sicilian Mafia clan led by patriarch Vittorio (Jean Gabin), which includes his sons Aldo (Yves Lefebvre), Sergio (Marc Porel), and son-in-law Luigi (Philippe Baronnet). While in prison, Sartet shared a cell with an electrician (Christian de Tillière) who installed an extensive security system at a diamond exhibition in Rome. The electrician is in prison because, when he returned home early, he caught his wife in bed with a lover and shot them. Sartet offered a sympathetic ear and the electrician supplied him with the complete security system details.
Vittorio and a fellow Mafiose, Tony Nicosia (Amedeo Nazzari) of New York, visit the exhibition but find that additional security has been added that make what would have been a simple robbery very difficult. When they purposefully set off the alarm system, they find the local police station is just down the road from the exhibition hall. Nicosia conceives of an alternative plan to steal the diamonds while they are in transit to another show in New York. He sends Jack (Sydney Chaplin), an alcoholic, to France to inform Vittorio with the details.
Commissaire Le Goff (Lino Ventura) pursues Sartet with unbridled determination. Sartet killed two of his men in cold blood during an earlier arrest. Guessing that Sartet needs false papers in order to leave the country, Le Goff's enquiries lead him to the Malanese and their arcade game business which serves as a cover for their more illegal activities. While he questions Vittorio, Sartet slips out of the building in a car, right under Le Goff's nose.
Jeanne (Irina Demick), wife of Vittorio's son Aldo and an able crook in her own right, becomes increasingly fascinated by Sartet. She has always felt out of place as the only French person in the Sicilian clan. While hiding out in a villa near the Italian border she attracts Sartet's attention by sunbathing nude but as they kiss they are caught in the act by Luigi's six-year-old son Roberto (César Chauveau). Jeanne gets the boy to promise not to mention it to anyone.
In Rome, the gang subtly kidnap Edward Evans, the insurance man sent to oversee the transfer of the diamonds to New York. Sartet takes his place and joins the other officials accompanying the diamonds on a regular scheduled flight to New York via Paris. Among the passengers joining the plane in Paris are Jack, Jeanne, Vittorio and his sons. Things almost go wrong when Evans' wife (Sally Nesbitt) turns up and even boards the plane looking for her husband, but Vittorio leads her to believe that her husband will be on another flight.
Having tried to contact her husband's hotel in Rome and being told that he has left, Mrs Evans goes to the police. At police HQ, she identifies Sartet as one of the men she saw on the plane while it was grounded in Paris. Told that his enemy has left the country, and guessing what he is up to, Le Goff requests a cigarette, having given up smoking some time ago.
Meanwhile, the plane is making its descent towards New York when the gang suddenly hijacks the aircraft. The crew are held at gunpoint and Jack, a former pilot, takes over the co-pilot's seat.
Warned of Sartet's imminent arrival in America, the local police race to the airport, but the plane in fact overshoots New York City and lands on a freeway which has been closed off by the local mob. Other Mafia men are waiting in cars. They unload the diamonds from the plane and split up, Jack for Canada and the Manalese for Paris. Intending to move to Veracruz, Sartet hides out in New York while awaiting his share of the proceeds.
Back home, late one evening, the Manalese are watching a film on TV which includes a scene of a couple kissing on a beach. Roberto says that it "Looks like Auntie Jeanne with Mister Sartet". Jeanne denies this but the others will not accept it. They lure Sartet back to Paris by withholding his share of the loot. Jeanne calls Sartet's sister Monique (Danielle Volle) to warn him that he is walking into a trap. Monique goes to the airport but when her brother fails to turn up she is confronted by the Manalese boys who are subsequently arrested by Le Goff and his men who had Monique under surveillance.
Sartet actually came by an earlier plane. He contacts Vittorio, demanding his share. They meet at a place outside of town where Vittorio shoots both Jeanne and Sartet dead. He then returns home, only to be arrested by Le Goff.