Air Force pilot Steve Ross [Tom Neal] returns to San Francisco after a bombing raid on Tokyo, where he clings to memories of his lost love, Abby Drake [Barbara Hale], whom he believes perished on duty in Bataan. His leave is cut short when orders pull him to Washington for a top-secret mission. Because Steve grew up in Japan and speaks fluent Japanese, the army asks him to undergo plastic surgery and become a Japanese officer, so he can infiltrate enemy lines, contact scientist Lewis Jardine [Marc Cramer], a prisoner of war who holds the formula to build an atomic bomb.
After the operation, he becomes Tomu Takishima, a Japanese soldier being shipped back toward Tokyo. As his transport nears the city, he makes a daring jump overboard, swims ashore, and makes his way to Kamuri concentration camp where Jardine is housed.
At the camp, Steve is assigned to the supply depot, where he meets Haan-Soo [Keye Luke], a Korean boy who works with the underground. Haan-Soo tells him Jardine is hospitalized with black fever, so Steve bribes Maj. Nogira [Leonard Strong], the medical director who has been pilfering hospital supplies, to get reassigned to the hospital. The decision must be approved by Col. Hideko Okanura [Richard Loo], the perceptive camp commander who was also Steve's college roommate; Okanura fails to recognize him at first, but suspects they've crossed paths before.
In the hospital, Steve encounters Abby Drake, who has been captured by the Japanese and is serving as head nurse. He contacts Jardine, who at first doubts that the man in uniform is truly American, but Steve convinces him.
Okanura then makes a move toward Abby; when she rejects him, he retaliates by targeting Nogira. Nogira is accused of stealing, demoted, and ordered to commit suicide; he does, and Okanura uses the death to accuse the nurses of murder, summoning Abby to his office. There, Okanura again tries to seduce her, but Steve arrives with proof of Nogira's supposed suicide, freeing Abby to leave. Wounded, Steve is whipped by the commander as Abby tends his wounds, and she begins to feel conflicted about him.
Steve then confesses to Jardine that he and Abby were once engaged, and that Abby still wears Steve's wings. The trio devises an escape plan, but the Japanese commander remains vigilant and has sent for the man who might verify Steve's identity. Jardine proposes a diversion: an explosion at the munitions plant. Haan-Soo departs to carry out the plant attack while Abby is summoned to a party at Okanura's house, creating a trap.
Under the guise of delivering an officer to the party, Steve drives to the commander's house with Jardine hidden in the back seat. Inside, Abby is kept in a room as Okanura prepares to assault her, and the sight of Steve in pursuit, chased by a watchdog, triggers a moment of recognition in Okanura, who realizes the truth about his roommate. He confronts Steve before the officers, but the munitions plant explodes, throwing everyone to the floor.
Steve overpowers Okanura, frees Abby, and finally reveals his true identity to her. With guards closing in, Steve, Abby, and Jardine flee toward the beach, where a rescue boat waits. Haan-Soo joins the effort, and Steve and Haan-Soo hold off the pursuing soldiers long enough for Abby and Jardine to reach safety. Their risky sacrifice helps spur the development and deployment of the atomic bomb, a pivotal moment intended to safeguard humanity.
> "mankind can walk unafraid in peace with goodwill toward men."
