[1] Leaving Paris for his summer vacation, the naïve Antoine Maréchal has his 2CV totally wrecked in a collision with the Rolls-Royce of company director Léopold Saroyan.

As compensation, Saroyan offers Maréchal the chance to drive a friend's 1964 white Cadillac DeVille convertible from Naples to Bordeaux, all expenses paid.

Unknown to Maréchal, Saroyan is the leader of a criminal gang and the Cadillac is filled with heroin, gold and precious stones, including the largest diamond in the world, the Youkounkoun.

After an accident, in which the repairman finds and removes the gold, Maréchal reaches Rome, where a pretty manicurist called Gina joins him.

Taking the Cadillac on the last leg to Bordeaux, he has another accident with Saroyan's prison transport, in which he finds the Youkounkoun and can thus expect a reward of 100 million francs coming to him.