Leaving Paris for his summer vacation, Antoine Maréchal, a naïve but well-meaning man, ends up with his modest 2CV totally wrecked in a collision with the Rolls-Royce of ambitious company director Léopold Saroyan. As compensation, Saroyan offers him the chance to drive a friend’s pristine 1964 white Cadillac DeVille convertible on a long, scenic road from Naples to Bordeaux, with all expenses paid. The catch is big and concealed: the Cadillac is secretly packed with heroin, gold, and precious stones, including the legendary Youkounkoun, the largest diamond in the world. Maréchal accepts the odd job, unaware that Saroyan is the head of a criminal gang and that both his own associates and a rival Italian crew are shadowing the car and its treasure.
In Naples, after an early accident, a repairman discovers and removes the gold, clearing part of the mystery but leaving the rest to unfold. In Rome, a charming manicurist named Gina joins Maréchal, ostensibly to offer a helping hand, but she does it to provoke jealousy in her fiancé. Her plans expand when she befriends a striking German hitchhiker named Ursula, who agrees to accompany them on the journey. As night falls, the Italian gang steals the Cadillac, triggering a chase with Saroyan’s group pursuing them. A gunfight erupts, and in the chaos the heroin is destroyed, with the criminals ultimately returning the car to Maréchal intact but much wiser about the danger he’s in.
The plot thickens when Mickey, one of the Italian crooks, seduces Ursula and invites her to join their gang. In a tense stand-off in an isolated location, Mickey makes a last attempt to eliminate Maréchal, but Ursula, resourceful and quick-thinking, sabotages the car’s battery, preventing the theft and saving Maréchal’s life. With the immediate threat neutralized, Maréchal salvages a new battery and discards the ruined one, which hides a stash of diamonds—now useless to the thieves and dangerous to hold.
As they cross into France at Menton, Maréchal witnesses Saroyan’s car being stripped by the police, and he realizes the authorities are pursuing the same vehicle, suspecting foul play on the part of Saroyan. Determined to reach Bordeaux, he heads toward Carcassonne and calls an old friend who has become chief of police there. Even as they stay just ahead of their pursuers, the party is driven toward a final confrontation. Maréchal lures the criminals into the unmarked side door of the police station, and they are arrested, their grand plan foiled for good. Maréchal then drives the Cadillac onward to Bordeaux for the last leg of the journey, only to collide again with Saroyan’s prison transport. In the wrecked Cadillac, he finally uncovers the Youkounkoun and senses the possibility of a massive reward—100 million francs—waiting for him.
The ride to the police headquarters ends with an unexpected, uneasy camaraderie forming between Maréchal and Saroyan, born from a shared brush with danger and a ridiculous, chaotic chain of events that brought them to this unlikely crossing of paths. The voyage, filled with misadventure, deception, and narrow escapes, leaves Maréchal with more than a fantastical story to tell—it changes how he sees luck, risk, and the odd luck of getting away with it all.
