French private investigator Claude Chavasse (Maurice Chevalier) discovers his client's wife has been having an affair with an American playboy, Frank Flannagan (Gary Cooper). When the client decides to kill Frank, Claude's sheltered daughter, Ariane (Audrey Hepburn), throws off the plan and saves his life. The two are instantly attracted to one another, but Ariane doesn't reveal her name. Frank then hires Claude to locate Ariane, unaware he has sent him on a mission to find his own daughter.