In Vienna, 1938, Katie O'Hara, a gold-digging ex-burlesque queen from Brooklyn posing as Phildelphia socialite Katherine Butt-Smith, is about to attain her dream of wealth and status by marrying Baron Von Luber of Austria.  On the eve of her wedding, Katie is visited by foreign correspondent Pat O'Toole, who is writing an article that will expose the Baron as a Nazi undercover agent.  Recognizing Katie as a Brooklyn stripper, Pat becomes infatuated with her, and when the Baron arrives to inform her that they are leaving immediately for Czechoslovakia, Pat follows her to Prague, where Katie and the Baron are married.

After the fall of Czechoslovakia, the Baron and his bride board a train for Poland.  When the Baron is questioned by the police about an illegal cache of money found in his wife's purse, Pat enters Katie's compartment to warn her about her husband.  Katie, content with her diamond ring and expensive jewels, refuses to listen, however, and sends Pat back to his compartment—alone except for his saxophone.

In Warsaw, Pat meets Katie at her hotel and invites her to a café, where he continues to lobby her to leave the Baron.  While Katie and Pat flirt with each other over drinks, the Baron offers to sell Polish General Borelski guns so that his countrymen can defend themselves against the Germans.  When the guns fail to work, Pat contacts the general, who has come to realize that the guns, which are useless, are part of the Baron's plot to destroy Poland.

Soon after, the general, who is the only person who can testify against the Baron, is assassinated, and the Baron is jailed for questioning.  After the fall of Warsaw, Pat visits Katie in her hotel suite, where she begins to discern the trail of destruction that her husband has wrought across Europe.  When the children of Anna, her Jewish maid, run terrified into her suite, seeking refuge from the Germans, Katie decides to take a stand and helps them escape the country by giving Anna her passport.  After learning that the Baron has been freed, Katie decides to leave her husband, and Pat arranges to have her name put on a casualities list to prevent the Baron from searching for her.  When the Gestapo demands Katie's and Pat's passports, Katie presents them with Anna's passport, and as a result, they are imprisoned as Jews.

Rescued by the American consul, they follow the Baron across Norway, Holland and Belgium to Paris.  In Paris, they visit Le Blanc's photo studio to have new passport photos taken.  After Pat leaves Katie at the studio to go shopping, Le Blanc, an American counter-agent, recognizes her as the Baron's wife and implores her to return to her husband to discover his plans for France.  That evening, Pat professes his love to Katie and proposes.  When Katie tries to tell him of her mission, Pat begs her not to break the magical spell of the night.

The next morning, Katie leaves Pat before he awakens.  While drowning his loneliness in a Paris café, Pat discovers that the Baron is seated at the table next to his, dressed in a Nazi uniform.  The Baron informs Pat that his wife has returned.  He confides his suspicions about her constant questions, and threatens to turn her into the secret police unless Pat agrees to broadcast German propaganda into the United States.  After the Baron departs, Le Blanc, who is seated at the next table, suggests that Pat accept the Baron's offer and then double-cross him.  Later, Katie visits Le Blanc's studio with a roll of film that she has shot of her husband.  In one of the Baron's crossword puzzles as captured on film, Le Blanc discerns a Nazi secret code.

After he sends the code to U.S. intelligence, the Nazis break into the studio, shoot Le Blanc and arrest Katie.  Katie is imprisoned in her hotel suite, where Anna, now a maid at the hotel, helps her escape by giving her a maid's uniform.  Meanwhile, at the radio studio, Pat has taken Le Blanc's advice and alters his speech to portray the Baron as dangerously power-hungry.  After the embarrassed Baron is arrested by the suspicious Gestapo, Katie joins Pat at the studio, and the pair board a ship bound for America.  As Pat leaves to speak to the ship's purser, Katie is approached by the Baron, who informs her that he has been pardoned by Hitler.  Fearful that he plans to continue his destructive ways in America, Katie threatens him, and they begin to struggle along the railing of the deck.

Rejoining Pat on the deck below, a shaken Katie recounts how she pushed the Baron overboard in self-defense.  When Pat informs the captain that a man is overboard, the captain turns the ship around, but when Katie informs him that the Baron can't swim, the captain abandons his search.