Young cello student (Audrey Hepburn), daughter of a private detective (Maurice Chevalier), falls in love with one of the persons under surveillance, the American playboy Frank Flannagan (Gary Cooper), who is widely liked by the ladies in Paris. Arianna saves Frank from a jealous husband and gets to know him better. She introduces herself like an experienced woman and she plays that role so good that Frank asks a detective (the girl's father, but he doesn't know it) to investigate her past...