Au voleur! (1960)
Directed by Ralph Habib

Film Synopsis

Commissioned by someone he has never met, thief Serge Duval sets out to steal the famous Nabab diamond, which is currently in the possession of a diplomat named Demosthène.  Assuming the name Count Amédée Fornari, Duval gains admission to the luxury Riviera hotel where the diplomat is staying.  The latter is accompanied by his young protégée, Amenita, whose only task is to wear the diamond around her neck.  With a perfect reproduction of the Nabab in his pocket, Duval attends the hotel's annual ball, where he intends to substitute the real jewel for the forgery while dancing with Amenita.  Alas the operation fails.  Meanwhile, tired of being Demosthène's object, Amenita steals the diamond herself.  When the diplomat notices the jewel's disappearance, Amenita hides the Nabab in Duval's pocket, heedless of the maelstrom of confusion that will soon ensue...