Rick Leland, a captain in the United States Coast Auxiliary Army, is found guilty of stealing regimental funds and is dishonorably discharged from the service. He subsequently tries to enlist in the Canadian artillery but is turned down because of his record. Disillusioned, Rick buys passage on the Japanese freighter, the Genoa Maru , intending to offer his services to the Chinese. Traveling on the same ship is Alberta Marlow, who is heading for Panama, and Dr. Lorenz and his Japanese servant, T. Oki. Lorenz is a student of Japanese culture and is interested to learn that if war broke out in the Pacific, Rick would not participate. Over drinks, Lorenz quizzes Rick about his experience in the artillery in Panama. Rick also spends some time flirting with Alberta, who returns his interest. When the ship docks in New York City, Rick visits a man who turns out to be his undercover contact. Rick, whose dishonorable discharge was faked to cover his investigation of Lorenz, asks his contact to inquire into Alberta's background, as well. When Rick rejoins the ship, another passenger, Joe Totsuiko, a Nisei, joins the company. On board, Rick prevents a Filipino man from shooting Lorenz, who explains that some Filipinos resent his ties with the Japanese. Alberta later calls Rick's attention to the fact that Lorenz has a new servant who is using the same name as his former servant. Eventually Lorenz offers Rick money to disclose military information and Rick agrees. In Panama, the passengers learn that the ship will not be allowed to travel through the canal. After disembarking, they all check into a hotel. Lorenz demands that Rick find out the schedule of airplanes flying over the area in return for the money he paid him earlier. Rick also learns that Alberta is not who she is pretending to be and confronts her, but just as she is about to explain, she is called to the telephone. Rick then searches her room, where he finds that Lorenz has earlier done the same. Lorenz warns Rick about Alberta before knocking him unconscious. When Rick comes to, he alerts his contact about Lorenz' plans, but the man is killed before he can act on the warning. Sam, the hotel keeper and an old friend of Rick's, puts him in touch with a man who advises him to travel quickly to a nearby plantation. There, workers are loading a bomber under cover of darkness. Rick is captured and taken inside, where he discovers Alberta and Joe. The owner of the plantation is Alberta's father, who has been forced to provide a cover for the Japanese. Lorenz's servant turns out to be a bomber pilot. Now the Japanese, supported by Joe and Lorenz, plan to bomb the canal's locks. Hearing the plane start its engines, Rick initiates a fight, during which Alberta's father is killed. Rick escapes and shoots down the plane, killing the Japanese pilot. His plan a failure, Lorenz tries to kill himself, but cannot go through with it. Rick captures him and takes him in for questioning. Alberta, who has been loyal all along, accompanies Rick.