During World War II, American G-Man Tom Hanley is assigned to snare suspected spy and lounge singer Carla Nillson in London. Though Carla claims to be a Norwegian war refugee, Tom suspects that she is behind a sabotage effort aimed at destroying the shipment of American war planes to England. Posing as an attorney doing business with the U.S. Consulate in London, Tom finds Carla at a nightclub, and takes cover with her in a bomb shelter during an air raid. Carla later asks him to help her get a visa for the United States so that she can perform there. As Carla and Tom become fast friends, they are joined by Reggie Oliver, a Scotland Yard detective posing as a music critic interested in Carla's singing career. Though Tom and Oliver eventually discover that they are spending time with Carla for the same reason, they disagree about how to best trap the suspected spy. Tom, believing that Carla would be more easily apprehended in America, procures a visa for her and quietly escorts her to Lisbon to catch a boat to New York. Oliver, however, finds the couple in Lisbon, and rejoins them, claiming that he is being been sent to America by his newspaper to cover her arrival there. By the time they arrive in New York, Tom falls deeply in love with Carla. Soon after arriving in New York, Carla performs a concert at the Long Island home of candy tycoon Sidney Grenner. It soon becomes evident, however, that Mr. Grenner is actually the head of the sabotage ring, and is using the party as a front for planning their biggest operation yet: the destruction of every American cargo ship leaving for England. During Carla's radio broadcast recital, Tom and Oliver notice an unusual pattern in her singing and suspect that she is singing in code for the enemy. After discovering the code on a piece of paper in Grenner's bedroom, Tom tosses it out the window to a waiting FBI agent. Carla later discovers that Tom is a government agent, and though she still carries a torch for him, she tells Grenner about him. Much to Carla's dismay, Grenner orders her to lure Tom to a hotel rooftop garden, where a sniper will be waiting to shoot him. Carla leads Tom to the garden in the middle of a Fourth of July fireworks display, but just as the sniper fires his gun at Tom, she jumps in front of him and takes the bullet herself. After the secret code is deciphered by Oliver and the FBI, the sabotage ring is shut down and its leaders are arrested. Carla, meanwhile, is visited in her hospital room by Tom, who is duty-bound to arrest her. His love for Carla still alive, Tom promises a happy future for the two of them after the war.