In post-World War II Italy, fourteen-year-old Mario, the only remaining member of a pack of wild children, watches as the aristocratic Luca Rossi argues about money with a man named Giorgio and then bludgeons him to death with a tire wrench. After Luca drives away, Mario takes the wrench and buries it near the lumber shed in which he lives. Several days later, Mario goes to the elegant Rossi home and tells Luca and his father Emilio that he witnessed the murder. Luca insists he acted in self-defense, but Mario refutes Luca's claim and announces his intention to live in the house with the Rossi family. When Luca threatens him, Mario warns that he has left an incriminating letter with a friend, who has been instructed to take it to the police if Mario is not seen for a few days. Emilio resigns himself to the arrangement, and Mario moves in. That night, Emilio and Luca tell Luca's wife Dina what has happened, and Luca bitterly laments his once-wealthy family's diminished circumstances. Mario gradually grows accustomed to living with the family, and Emilio enrolls the boy in school and attempts to improve his table manners and grooming habits. One night, Luca comes in while Mario is doing his homework, and realizes the nearly illiterate boy could not have written an incriminating letter. The next day, Luca follows Mario to the lumber shed and watches as he digs up the wrench. Mario then encounters police inspector Umberto Spezia, who questions the boy about his new living arrangements. Later, after Mario has returned to the palazzo and hidden the wrench in the stable, Spezia comes by to check out his story, and Emilio claims that Mario is the orphaned son of an old schoolmate killed during the war, adding that he plans to adopt the boy. Mario is thrilled, but after Spezia leaves, Emilio admits he does not know why he said that. Enraged, Mario makes a crude pass at Dina, and when Luca finds out about it, he beats Mario savagely until Emilio intervenes. Dina asks Mario's forgiveness, and the boy breaks down and sobs as she comforts him. Later, Emilio tells his family they will take a seaside vacation at Bellamare, but as they are about to leave, Spezia arrives with the news that Emilio's friend, who supposedly was Mario's father, is actually alive and living in Rome. Spezia says he suspects that Mario's presence in the Rossi household is somehow connected to Giorgio's murder, and forbids the family to leave the city. Luca tries to convince Mario that he is now his friend, showing him the tire wrench and, as a gesture of trust, asking him to hide it again. Mario softens, and he accepts Luca's offer to sneak out to the seashore overnight for a swimming lesson. At Bellamare, Luca takes Mario out in a rowboat, and when they are far from shore, he throws the oars overboard and bashes a hole in the bottom of the boat. Leaving Mario in the sinking boat, Luca swims back to shore and calls Emilio to say that Mario took the boat out alone and has disappeared. The Rossis call the police and head to Bellamare, just as an unconscious Mario is discovered on the beach by two fishermen. Spezia insists on interrogating the delirious boy, but Mario refuses to say anything that might hurt Emilio. Shamed by the boy's loyalty, Emilio tells Spezia that Luca is a murderer. Later, on the beach, Emilio embraces Mario as his son.