Adapted from a play by Hans Szekely and P. A. Stemmle, a woman steals a pearl necklace and smuggles it out with an American but falls in love with him.
In Paris car engineer Tom Bradley (Gary Cooper) is given two weeks vacation by Gibson (William Frawley) and drives with a sign. Madeleine de Beaupre (Marlene Dietrich) orders an expensive pearl necklace from Aristide Duvalle (Ernest Cossart) to be delivered to her husband Dr. Pauquet (Alan Mowbray); but she tells him she is the wife of jeweler Duvalle, who she says is demented and will present a bill. Duvalle tells her he will take it back if she changes her mind. She leaves while Dr. Pauquet treats Duvalle as a patient until they learn neither is married.
Tom drives and stops for a photo, but Madeleine's car splashes him. Her horn gets stuck, and he fixes it. After she passes him in dust, he reconnects the horn. She asks his help, and he fixes it again. At customs inspection she puts the necklace in his coat. Tom is fined for hiding cigarettes. She asks his help again and gets a ride. When Tom puts on the wrong coat, she takes his car. Tom finds his car later wrecked. At a hotel Carlos Margoli (John Halliday) calls on Madeleine and asks for the pearls. A policeman (Akim Tamiroff) tells her an American had his car stolen. Tom comes in. Carlos says his niece Madeleine will be disowned. She apologizes and explains to Tom. The three drive to an inn. Madeleine wears imitation pearls. After dinner she sings "Awake In a Dream." Carlos does magic tricks and gets the pearls for Madeleine. Carlos warns her to go to bed soon. She acts sleepy as he declares his love. She hears police sirens and tells Carlos she is afraid. In the moonlight Tom kisses Madeleine.
Carlos wakes Madeleine, takes the necklace, and says he will take Tom to town; but she urges Tom to stay. Carlos goes alone to Madrid. Tom and Madeleine have a romantic breakfast. A week later she finds Olga (Zeffie Tilbury) has arrived. Olga says they can sell the pearls in Madrid. Carlos appears and demands Madeleine sell the pearls. Madeleine says she is quitting being a countess. Olga dares her to tell Tom about herself. Tom shows Madeleine he booked the bridal suite on a ship and corrects lies he told her. Madeleine says they are from different worlds but admits she stole the necklace, saying good-bye. At dinner Tom implies he spanked Madeleine to give her another chance. They discuss possible war and America. Tom asks for the pearls, but Carlos pulls a gun. Tom kicks it away and demands the necklace. Carlos runs out. Madeleine and Tom return the pearls to Duvalle and plan to marry. Madeleine gets parole, and they wed.
Amid a clever jewelry heist and romance of opposites a brief dialog indicates awareness of likely war that will drag America in to settle it. The story implies that the strong American will triumph over the criminal Europeans.