Bronek, Horst Buchholz, a young Polish sailor, lands in Cardiff with plans to propose to his girlfriend Anya, Yvonne Mitchell who is living far from home as a Polish exile. He quickly discovers Anya is not where she’s supposed to be; instead, a dockside prostitute occupies her flat. The landlord reveals Anya left weeks ago and owes back rent, even though Bronek has been sending monthly payments. Determined to reach her, he pays off Anya’s debts and secures her new address, hoping to bridge the distance between them.
Meanwhile, Gillie Evans, a wily eleven-year-old orphan who runs the streets with a rough edge, navigates the bustling docks with a fearless air. She’s not afraid to test boundaries, and her footprint on the world is as loud as the clanging cranes and creaking tenements around her. Bronek, in search of Anya, enlists the help of the dockside kids, and Gillie becomes his guide as they move through a crowded, gritty landscape toward a place that might hold Anya’s new life.
When Bronek tracks Anya to a new flat, she coldly instructs him to leave. Jealousy flares as Bronek learns that Anya has been seeing a married older man, a sportscaster named Barclay, Anthony Dawson. In a volatile confrontation, Anya pulls a gun, but Bronek wrests it away and fires, killing her in a terrible moment of rage and betrayal. Gillie, who had been playing in the staircase with a makeshift toy, witnesses the murder by peering through the letterbox, though the noise of her “toy bomb” had masked the real gunshots from others in the building.
Gillie’s instinct for risk drives her to seize the gun, hoping to gain status among the neighborhood kids. Barclay arrives after the deed, but the sight of Anya’s body sends him fleeing. A neighbor discovers the body and summons the police, setting in motion a tense investigation by the city’s authorities.
The case lands on the desk of Superintendent Graham, who quickly understands that Gillie was inside the hall at the moment of the shooting and thus may hold crucial knowledge. Yet Gillie, a habitual liar, offers a distorted account of the events and withholds the gun’s location. Bronek, who has seen Gillie take the weapon, slips away into the shadows of the tenement’s attic, and in a surprising turn of mercy, he pursues a tenuous bond with the girl, agreeing to take her to sea if he can flee the country.
Bronek’s plan crystallizes around departure on a Venezuelan merchant ship, the Poloma. The pair retreat to a countryside hideout for the night, where Bronek entertains Gillie with reenactments of his overseas journeys. The investigation widens when a photograph of Bronek and Anya surfaces, drawing more attention to his identity. Barclay becomes a leading suspect after admitting he visited Anya’s apartment on the day of the murder.
With the Poloma poised to sail, Bronek convinces Gillie to let him go alone. He entrusts her with his metal cigarette case as a symbol of their fragile alliance. Picnickers stumbling upon the hideout bring Gillie to the police, where she continues to lie, pointing blame at Barclay as the killer. Yet when Graham and Gillie replay the crime for the inspector in Anya’s flat, she blurts out that the killer is Polish, despite still denying Bronek’s central involvement.
The authorities realize Bronek is the culprit, and the sense of jurisdiction becomes a race against the clock: the ship’s three-mile boundary marks the limit of British law. Graham drives Gillie to Barry Docks and to a pilot boat that will take her to the Poloma as the vessel edges toward territorial waters. Gillie’s stubbornness to stay with Bronek surfaces, and she tries to throw the investigators off by clambering aboard. On the ship, Bronek and Gillie deny any association, but the captain blocks an immediate arrest, arguing that the ship’s navigator has charted the Poloma’s position just outside the three-mile limit, thus beyond British jurisdiction.
Gillie dashes about the deck, attempting to stay hidden, and in a final act of desperation she falls overboard while trying to stow away on the ship in hopes of remaining with Bronek. Bronek dives in to rescue her, losing his ship in the process. The pair are saved by the police boat from Barry Island, bringing them back to safety. Bronek, moved by Gillie’s embrace, confesses his guilt, and Graham acknowledges his bravery in saving the girl. The Poloma sails on, and Bronek’s chance at freedom disappears with the sea air.
