On a quiet boating holiday along the south coast of England, Simon Wells, a middle-aged American who has recently separated from his career as an insurance executive, meets Joan in Weymouth. She lures him into a brutal mugging led by her brother, King, and his motorcycle gang. Though the assault ends with no lasting physical harm, it sets off a tense undercurrent between the two travelers and the people who surround them.
The following day, Joan joins him on his boat and defies her overprotective brother, who tries to keep her from leaving. Simon is willing to forgive and move on, while Joan explains the abuse she suffers whenever men show interest in her. The fragile balance between desire, danger, and coercion pushes them toward an awkward alliance as they drift along the coast.
Meanwhile, a far more unsettling reality unfolds in the caves along the nearby shore. There, nine children, all aged eleven, live with skin that feels cold to the touch. They are healthy, well-dressed, and clearly intelligent, yet they know little about the outside world. Their home is under continuous video surveillance, and Bernard — played by Alexander Knox — directs their education through a closed-circuit system, deflecting questions about their isolation with promises that they will learn the answers someday. The children are regularly visited by men in radiation protection suits, a reminder that something dangerous lurks just beneath the surface of their existence.
That night, Joan and Simon share a kiss and then return to a cliff-top house where their tryst continues, even as King’s gang encircles the property. They escape and reach the relative safety of a nearby military base. From there, they descend a cliff to a secluded beach and discover a network of caves that lead to an underground bunker attached to the base, where the children live.
Inside the caves, Bernard is forced to keep the children under constant watch, yet he allows them one chamber free of cameras. The children themselves are largely unaware that their secret hideout is known to their captors. They cherish mementos of people they believe are their parents and welcome Joan, Simon, and even King into their space, secretly smuggling food to them. The trio realizes the risks but feels a moral pull to rescue the children, even as they begin to feel unwell.
Bernard pressures the children to surrender their new friends and reveals his knowledge of the hidden place. The children refuse and destroy the surveillance cameras. Bernard then sends men in radiation suits to reclaim the intruders, but King and Simon overpower them. Using a Geiger counter, Simon discovers that the children are radioactive, a consequence of a nuclear accident that has altered their biology.
The escape becomes a chase through the caves and beyond. The intruders lead the children out, but more men in radiation suits close in, and most of the children are dragged back to the bunker. King seizes one of the boys and escapes in a stolen car, only to succumb to radiation sickness; the boy is recaptured. A helicopter pursues them as the vehicle careens, and King is killed in the ensuing crash.
Joan and Simon manage a perilous escape by boat, but both fall ill from the radiation. A helicopter hovers overhead, prepared to destroy the survivors if they die and fail to surface. The men in charge want the story to end with the couple dead, but the beachgoers below remain oblivious to the desperate drama unfolding behind them.
In a final twist, Bernard confides in his mistress Freya that he regrets the moment the children learned they are prisoners. The children were born radioactive as a result of a nuclear accident, granting them resistance to fallout and, in Bernard’s view, a purpose to endure a future war he believes is inevitable. When Freya rejects his plan, he kills her. The closing shot returns to the holiday beach, where life goes on for those carefree visitors, completely unaware of the hidden, radioactive children nearby and their quiet, desperate pleas.
