Despite the title, the film is not about the Mafia. The Sicilian clan in question is the Manalese family, led by patriarch Vittorio (Gabin), who helps jewel thief Roger Sartet (Delon) escape from prison -- Sartet is given a miniature circular saw to cut his way out of a paddy wagon during a transfer -- in return for the plans for the security system for an upcoming jewelry exhibition in Rome, which Sartet acquired from the designer, now a fellow convict. Sartet wants in on the prospective robbery but Vittorio doesn't trust him because Sartet is a killer -- and an outsider. After Sartet has to shoot his way out of a tryst with a prostitute, Vittorio keeps him under close wraps until he has a chance to case the Rome site himself, in the company of old criminal pal from America. They discover that the security system is more extensive than Sartet had indicated, and pretty much unbeatable. Here the film shifts direction. Instead of a Rififi-style caper, Sicilian Clan goes ultra-modern when Vittorio's American buddy gets the idea of the Manaleses hijacking a plane carrying the jewels from Europe to the U.S. The American can secure an impromptu landing strip for the captive plane by appropriating a stretch of highway outside New York City. The caper becomes a matter of getting the clan (and Sartet) on board the plane without the police (led by Ventura playing like the French Walter Matthau) noticing the highly-wanted Sartet, then pulling off the hijacking without the U.S. Air Force blowing the plane out of the sky. All goes well until Vittorio learns about Sartet's beach affair with one of his daughters-in-law and decides that the randy Corsican should die. The old man's brutal assertion of patriarchal authority proves the undoing of the entire gang.
Sicilian Clan has some moments of intelligent suspense, particularly after Sartet boards the plane in the guise of a British security agent, when his cohorts have to deal with the sudden appearance of the real man's wife at the airport. After discovering that her husband is not on the plane, Vittorio tries to throw her off the trail by explaining (using an airport phone, he pretends to be a government offiical) that the man is still at his hotel. Now she wants to call him at the hotel, and it becomes a race against time to get the plane off the ground before she realizes she's been tricked. Verneuil is at least efficient, but to what purpose? It seems like some too-careful balance was struck between Gabin and Delon, with Ventura's flic the odd man out, so that Delon disappears from a large section of the film while Gabin visits Rome. Neither star dominates the film long enough for audiences to identify with one or really understand what they stand for. When Vittorio resolves to destroy Sartet, is this a vindication of old-school values or the self-destructive outburst of an obsolete old man? Is Sartet the wild beast Vittorio thinks he is? We don't see enough evidence to damn him so, nor does the film really make a case that Vittorio is dangerously old-fashioned or simply irrational. The plot ends up looking contrived to set up an intergenerational showdown, reducing the film to an overcooked potboiler. It can probably be enjoyed on its own undemanding terms, but the best French crime films have spoiled me. I expect more of an immersively existential experience along the lines of Melville's Le Samourai, or Claude Sautet's Classe Tous Risques -- but sometimes a caper is just a caper. Dial down your expectations and Sicilian Clan may still satisfy.