Paolo, an Italian tour bus driver living in Paris, has just summoned up the courage to propose marriage to his German girlfriend Greta. However, a chance encounter with a French woman on a bicycle the very next day turns Paolo's life upside down.

John Arnold DeMarco is a man who believes he is Don Juan, the greatest lover in the world. Clad in a cape and mask, DeMarco undergoes psychiatric treatment with Dr. Jack Mickler to cure him of his apparent delusion. But the psychiatric sessions have an unexpected effect on the psychiatric staff and, most profoundly, Dr Mickler, who rekindles the romance in his complacent marriage.