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During a dinner party at her family home in Paris, Marianne Jannetier is warned by her politician fiancé Andre Benoit, that the city is about to be overtaken by the Germans, and that she and her mother must make their way immediately to Bordeaux.  Along with hundreds of others that night, Marianne and her mother evacuate Paris, but are caught in a German bombing raid.  Marianne's mother dies of a heart attack, so the distraught young woman decides to return alone to her Parisian home.

Meanwhile, a young German pilot, unwilling to kill innocent French women and children with his bombs, abandons his plane and parachutes into France.  The German soldier is mistaken for a British flyer by Lt.  Nicholas Jordan, a Texan flying for the British Royal Air Force, and his British gunner, Bruce McAvoy.  Nick takes the young German prisoner, but is later forced to land his plane in the French countryside.  In the meantime, the British are compelled to evacuate France, with Nick and McAvoy being inadvertently left behind.  Marianne returns to Paris to find that a French underground meeting is being held in her home as German soldiers march into the city.

She joins the underground, and is sent to work as a pianist at La Coquine, a waterfront café, where she sends coded radio messages through the piano.  In the meantime, Nick and McAvoy discover a German air fleet hidden in some stacks of hay.  They set fire to the bales, which signals a British aerial attack.  McAvoy is killed, however, as the two attempt their escape.  At the same time, Gestapo agents trace short-wave radio messages to La Coquine, so their commander, Hauptman Schwabe, goes there to investigate.

Nick arrives at the café a short time later, and he immediately comes under Schwabe's suspicion.  The Gestapo officer, rather than arresting everyone, decides to keep the café and Nick under close observation.  While Marianne wants to help Nick, both bartender Mouche and café proprietor Colette suspect him of being a double agent.  The next morning, the head of their underground unit, Professor Marceau, orders the café workers to turn Nick over to the Germans.  Just as the Gestapo arrives, however, Marianne realizes that Nick is really an American and hides him, and then the two are taken prisoner by Marceau.  After being questioned by the chief of the underground, Marianne learns that Andre is now a high-ranking Nazi sympathizer within the Vichy government, and she is ordered to spy on her ex-fiancée.

While Andre and Marianne are reunited in a hotel suite, Schwabe arrives and demands Andre's secret papers.  When the French politician refuses, Schwabe insists on leaving his counterfeit papers behind, as a "mouse trap" in case Marianne is truly a spy.  Later that night, Marianne falls for the "bait," and she is discovered by Andre, who then defends his association with the Nazis in hopes of convincing her to join him.  When he threatens to turn her over to the Gestapo if she refuses, Marianne kills Andre with his own gun.  As she makes her escape, a captured Nick is interrogated by Schwabe, who then learns of Andre's assassination.

Unable to gain admittance to underground headquarters, Marianne goes back to La Coquine, where she is met by the just-escaped Nick.  As the Gestapo surrounds the café, Marianne sends a message to London requesting assistance.  A British commando unit is then sent to the Paris waterfront in a captured German transport.  They arrive at the café just as Schwabe and his men are placing Marianne and the others under arrest.  Marianne, Nick and the other underground agents, along with the captured German officers, then board the seaplane and fly back to London and freedom.