While leaving his flat in Paris for vacation, Antoine Maréchal has a car accident. His 2CV is totally wrecked by Léopold Saroyan's Bentley, the director of an import-export company. As a compensation for the accident, Saroyan offers Maréchal to drive his best friend's 1964 Cadillac DeVille from Naples to Bordeaux, all expenses paid. Unknown to Maréchal, Saroyan is the leader of a criminal organization and the Cadillac is filled with drugs, stolen gold and diamonds, including the largest diamond in the World, the Youkounkoun. Maréchal departs for his destination, oblivious of both his cargo and Saroyan who is discreetly following him to watch over the delivery.
Unfortunately, a gang of Italian criminals has become aware of Saroyan's plan and follows the car closely, stealing it at every opportunity they get. Along the way, most of the illegal goods hidden in the Cadillac are lost or stolen by other parties. When Antoine Marechal pieces together Saroyan's plan while crossing the border, he turns the tables on both Saroyan and the Italian gangsters. This all comes to fruition in the French town of Carcassonne.