In Lisbon during World War II, the mysterious Deresco operates an opulent casino bearing his name, and many of his guests are spies from various countries, all seeking to buy or sell information, or even kill for it. Deresco himself does not care whether the Allies or the Axis nations profit from his espionage activities, as long as he receives the highest possible price. One evening, Deresco interviews famed Czechoslovakian dancer Maritza, who needs his help in obtaining passage to the United States. Deresco, who does not know that Maritza is a counter-spy working undercover to topple his organization, agrees to help her but warns her that she will be completely under his control. That same night, the American Clipper from New York arrives, and pilot Bill Flanagan is contacted by war correspondent and former prisoner of war John Craig, who is desperate to leave Lisbon with a roll of microfilm containing information about Japanese activities in Burma. While Flanagan sends Craig into hiding, Deresco's major domo, Blanco, picks up spy Alexis Vanderlyn, who was also on the Clipper. Vanderlyn believes that he is in Lisbon to help track down Craig and turn him over to the Axis, but actually, Deresco has orders to kill the no-longer useful spy. Vanderlyn pretends to have needed information, and Deresco agrees to let him live in the casino's hotel for the time being. Flanagan then comes to the casino to watch Maritza perform, and when they go out on a date afterward, he inadvertently reveals Craig's hiding place to her. Blanco is following the couple and informs Deresco of Craig's whereabouts, and later, when Maritza returns to Craig's secret cellar to try to gain his trust, she is followed by Deresco and his men, who drug Craig and take him to the prison-like tower at the top of the casino. Believing that Maritza betrayed him, Craig refuses to trust her, but during a flurry of activity in the casino, is able to tell Flanagan where the microfilm is hidden in the cellar. Vanderlyn, knowing that his time is almost up, tries to convince Deresco that he knows the location of the microfilm, but Deresco sees through his bluff and shoots him, then has Blanco throw his body from the tower to the river below. Soon after, Deresco's secretary Evelyn, who is jealous of Deresco's attentions to the lovely Maritza, helps Maritza and Craig escape from the casino. When they return to the cellar to retrieve Craig's microfilm, however, they are followed by the thugs and Deresco, who were told about the fleeing couple by Evelyn. Deresco finds the microfilm, but Maritza's compatriots arrive in time to capture the master spy and his minions. Later, at the airport, Maritza bids farewell to Flanagan and Craig, who are going to Washington, D.C. with the microfilm. Flanagan asks Maritza to accompany them, but she insists that she must remain in Europe to help the Allies, and Craig sadly agrees that if one is to live dangerously, it must be done alone.