In Paris, robber and murderer Roger Sartet [Alain Delon] escapes from custody with the help of the Manalese, a small, tightly organized Sicilian Mafia clan built around patriarch Vittorio [Jean Gabin] and his two sons along with his son-in-law. Sartet pays for his freedom with some valuable stamps and the clan hides him in an apartment above the arcade game manufacturing company they own as a front. Jeanne [Irina Demick], Vittorio's elder son’s French wife, looks after Sartet, but he slips away to visit a prostitute at a hotel and narrowly avoids capture by Commissaire Le Goff [Lino Ventura].
While Sartet was in prison, he learned about an engineer who worked on the security for a jewelry exhibition in Rome, and upon release he proposes a daring plan to the Manalese: rob the show. The clan is wary of the hot-headed outsider, so Vittorio and Tony Nicosia [Amedeo Nazzari], who has lived in New York for decades, travel to Rome to study the exhibition and its defenses. They notice enhanced security that could foil Sartet’s scheme, but upon learning the jewels will soon be moved to New York, Nicosia devises a plan to steal them in transit. He dispatches Jack [Sydney Chaplin], an alcoholic ex-pilot, to Paris to assist with the heist.
To ensure Sartet can leave with forged papers, Le Goff traces the forger who made his previous passport. Coincidentally, Vittorio has been using the same man to produce several fake papers, and Le Goff uncovers the studio’s number among Vittorio’s records. When questioned, Vittorio insists the employee no longer works for him, though the implication lingers.
The Manalese retreat to a hideout near the Italian border. Jeanne sunbathes nude in front of Sartet, and the two begin an affair, but their privacy is broken when Roberto, Jeanne’s six-year-old nephew, wanders in. She begs him not to tell anyone about what he’s seen.
Sartet travels to Rome and secretly kidnaps Edward Evans, an English insurance man, then takes Evans’ place among the officials guarding the jewels during their flight. In Paris, as Jack, Jeanne, Vittorio, and his two sons wait to board the plane, they are taken by surprise when Evans’ wife arrives, intending to accompany her husband to New York. Vittorio deftly manipulates her, steering her to believe Evans will be on the next day’s flight, while the jewels are actually set to move. As the plane loads and lifts off, Mrs. Evans calls the hotel in Rome and realizes Evans never arrived; she identifies Sartet as one of the men she saw on the plane.
During the plane’s descent toward New York, the Manalese hijack the aircraft. Police race to the airport, but Jack lands the plane on a new stretch of highway not yet open to traffic. Nicosia’s men are waiting to unload the jewels, and the gang splits up. Sartet hides in New York, waiting for his share and a ticket to Veracruz.
Back in Paris, the Manalese watch a film in which a couple starts to make love, and Roberto blurts out that it resembles what Sartet did with Jeanne. Although Jeanne denies the insinuation, Vittorio withholds Sartet’s share and lures him back to Paris. Monique Sartet [Danielle Volle] walks through the airport to warn her brother, only to find Vittorio’s sons and son-in-law awaiting Sartet; Le Goff arrests them.
Sartet, who had arrived in Paris earlier than expected, calls Vittorio to arrange a meeting at an isolated spot. Vittorio brings Jeanne along, and while Sartet is counting the money, Vittorio shoots both Sartet and Jeanne dead, leaving the cash beside the bodies. When Vittorio returns home, Le Goff is waiting to arrest him.
