As several boys play in the rubble of a bombed-out building in postwar London, young Erik Jenner finds a small handgun. Erik’s friend, Johnny Ashton, demands that the boy hand over the weapon to him, and in the ensuing struggle, the weapon fires and shoots Johnny. Thinking that he has killed the boy, Erik panics and flees. When a police ballistics test reveals that the bullet from the gun matches the bullet that ten years earlier had killed Capt. Moulton, an officer in the Criminal Investigation Division of the United States Army, Inspector McKenzie notifies Capt. Mark Andrews, a member of the CID and a friend of the deceased officer. After his superior officer reprimands Mark as a “tin soldier” for his sanctimonious attitude, Mark joins McKenzie and, having learned from the other boys that Erik fired the gun, accompanies him to question Erik’s mother Elsa. Elsa is incredulous when she learns that Erik has shot a boy and bristles when Mark indicates that he is more interested in finding the gun’s owner than her son. That night, when Erik nears his apartment building, he sees police waiting outside and runs away. When the boy fails to return home, McKenzie places a small article in the newspaper about the missing boy and gun, angering Mark, who fears the murderer will read the story and flee. The next morning, Elsa comes to Mark’s office as requested. Soon after, a man named Joshua Henry phones saying that he read the newspaper story and has seen Erik. Mark and Elsa then drive to Henry’s, where Henry lies that Erik stole a pint of milk from his doorstep that morning. While Mark and Elsa have their backs turned, Henry surreptitiously slips the pint of milk into his trashcan. Later, Erik goes to the restaurant at which Elsa works as a waitress and is ushered in by one of the other waitresses who offers him a sandwich while he waits for his mother to arrive. When a man asks to see the gun protruding from Erik’s pocket, the boy runs out of the restaurant just as Mark and Elsa arrive. Erik darts through the traffic, and although Mark tries to catch him ,the boy eludes him. Meanwhile, Henry goes to the bombed-out building, and pretending to be from Scotland Yard, offers to pay some local boys a penny a day for finding the gun. When two of Erik’s friends turn up at the Jenner house looking for him, Elsa questions them, and they tell her about the “penny man.” Upon learning about the penny man, McKenzie realizes that Erik is in real danger and decides to initiate a publicity campaign to find the boy. Mark asks McKenzie to give him twenty-four hours to find the killer before launching the campaign, and the inspector agrees. That night, as Erik hides outside the restaurant, waiting for his mother to finish work, Henry drives up and offers Elsa a ride home, then wangles an invitation to join her for dinner. Meanwhile, Mark questions dance hall hostess Vivienne Pascal, who was engaged to Moulton. The hard-boiled Vivienne is uncooperative until Mark offers to pay her for information, after which she invites him to her apartment later that evening. Before Mark arrives, however, Henry comes to the apartment and warns Vivienne to keep quiet. He leaves the building just as Mark appears, and Vivienne, now terrified, states that she "is dead" and breaks into tears. When Mark offers her a few words of sympathy, Vivienne asks him to kiss her, and afterward, as she turns her back to the window, a shot rings out, killing her. Spotting her assailant, Henry, running down the street, Mark follows him into a warehouse by the wharf. Without seeing Henry’s face, Mark fires at him, alerting a police officer. Just as Henry knocks Mark unconscious, the police officer arrives, and Henry jumps out the window and escapes without being identified. Soon after Mark revives, word comes that the body of a boy matching Erik’s description has been found in the river. Needing a positive identification, Mark accompanies Elsa to the morgue. Elsa breaks down when she realizes that the boy is not her son, causing Mark to apologize for his callousness and promise to bring Erik back to her. The next morning, Erik’s picture appears on the front pages of the newspapers and posters bearing his face are put up around town. That evening, Henry arrives at Elsa’s apartment, bearing steaks and caviar for dinner. On the street, meanwhile, the hungry Erik tries to steal a piece of fruit from a trucker. Taking pity on the boy, the man offers to buy him a sandwich. As Erik is eating, he hears a loudspeaker mounted on a truck announce that the Ashton boy is out of danger and Erik has nothing to fear. Relieved, Erik runs to a phone booth and calls his mother. Elsa is dining with Henry when Erik calls, and Henry immediately offers to pick up the boy. Elsa asks Henry to telephone Mark with the news, and as she goes to get their coats, he pretends to make the call. When they reach Erik, Henry asks about the gun and Erik states that he hid it in the ruins by his house. Meanwhile, Mark arrives at the Jenners’ apartment looking for Elsa, and the sergeant on guard outside informs him that she left with a man. As Henry drives Elsa and Erik toward the bombed-out site near their home, Elsa scolds Erik for stealing Henry’s milk. When Erik denies taking it, Elsa suddenly realizes that they are in danger and tells Erik to run. Jumping out of the car, Erik runs away and Henry leaps out to follow him. The driverless car then plunges over a wall with Elsa inside and crashes in the ruins. Hearing the crash, the sergeant and several bystanders rush to help. Meanwhile, Henry spots Erik hiding in the rubble and runs after him. Upon learning from the sergeant that Elsa has been injured, Mark hurries to her side where the barely conscious Elsa tells him that Henry is the murderer and that Erik is in grave danger. After Henry captures Erik, the boy lies that he hid the gun in the house. As Erik and Henry are climbing out of the ruins, Mark hears the boy whimpering and follows. He catches up to them at the top of the stairs, and in the ensuing fight, slugs Henry, sending him toppling over the wall to his death. Mark then walks Erik to Elsa.