When she learns a man's life is threatened, Ariane Chavasse (Audrey Hepburn) -- a young cellist inexperienced in amour -- poses as a world-wise femme fatale to foil a murder plot against American playboy Frank Flannagan (Gary Cooper) in director Billy Wilder's Paris-based romance. When Chavasse's mysterious alter ego captures Flannagan's heart, he unwittingly hires her father (Maurice Chevalier), a private detective, to learn more about her.