Set in a magnificent villa near a sun-drenched St. Tropez, lovers Jean-Paul and Marianne are spending a happy, lazy summer holiday. Their only concern is to gratify their mutual passion - until the day when Marianne invites her former lover and his beautiful teenage daughter to spend a few days with them. From the first moment, a certain uneasiness and tension begin to develop between the four, which soon escalates in a dangerous love-game.

Josette, the daughter of a hotel-casino owner on the French Riviera is wooed by Pradier, a young gambler she (and everyone else) mistakes for the Prince of Austravia. On the other hand another young man starts courting her and she does not feel insensitive to his advances. What she does not know is that Monsieur Lefèvre, whom she feels so much to her liking, is actually the real prince of Austravia. When the Austravian revolution dethrones the prince, the young man has only one citadel left to conquer: Josette's heart.

Story is primarily based on the adventures of Antoine, a young telegraph messenger on his second night of work. He is given three telegrams, one of which is for the president of the Camber of Deputies. Excited, Antoine runs his bicycle into a truck and loses the telegrams. What follows is an amusing and agonizing search for the missing messages. He is assisted first by a glum, wise-beyond-her-years little girl, Amelie; an off-duty police superintendent, a schoolmaster, a cookie-tin maker and a fireman. The harmonica musical theme is played by Flore Falvey.