While on vacation in Weymouth, American Simon Wells is beaten, robbed and left unconscious by a gang of leather-clad bikers after being led into a trap by a young woman.
Helped into a hotel by two military officers, Wells meets Bernard, a scientist working on a 'mysterious project' at the nearby Edgecliff Military Establishment, and Freya, an artist with whom Bernard was once apparently in a relationship. She is spending the summer at the Bird House, a cliff-top building adjoining Bernard's own house near Edgecliff, which she has turned into a studio.
The following day, on his boat in the harbour, Wells is approached by the young woman, Joan. The gang then arrive, and the leader, King, Joan's brother, threatens Wells and orders him to stay away from his sister. Wells sets off in his boat, and Joan, at his urging, jumps in, leaving King and the gang on the quayside. Putting ashore further along the coast, Joan leads Wells to Freya's Bird House, where, after breaking in, they help themselves to food and have sex.
The arrival of Freya forces them to leave by a window, and, having now been tracked down by the gang, they are chased into Edgecliff, tripping the alarms. Descending a cliff in an attempt to evade both King and the base's guards, they fall into the sea, from which they are rescued by a group of nine children, who help them into a cave via a door which they can open by moving their hands over a sensor.
The children are excited when they discover that Wells and Joan are warm; the children, in contrast, are ice cold to the touch. They explain that they are all 11 years of age and have always lived within the base, being taught by teachers who they only see via monitors. With the children hoping that they were their parents, Wells and Joan promise to try and help them escape. King, having climbed down the cliff in pursuit, also arrives in the cave after being rescued from the sea by one of the children.
Aware of the intruders, Bernard is anxious that the children should not watch them die, and tells them that he knows of the presence of the 'big people', and that it is imperative that they come out. Three military personnel in protective clothing enter the children's area, but Wells and King manage to overpower them, despite feeling increasingly weak and nauseous.
With a Geiger counter that one of the men had been carrying, Wells realises that the children themselves are radioactive not just their clothes, as he had first thought. Despite being warned that the children are dangerous, Wells decides to help them escape, using a key in the possession of one of the men to unlock a door shown to him by the children earlier.
This door is concealed in the rocks beneath the Bird House, and the children attempt to flee, only to be apprehended by security guards. One manages to escape with King in a car, but helicopters give chase and block the road. The child is recaptured, although the ailing King again manages to escape, only to be killed as he crashes the car into the sea. Wells and Joan, on the point of being put into an ambulance, are released by Bernard and allowed to return to the yacht.
Freya, having witnessed these events, is told by Bernard that the children had been born to mothers exposed to radiation in an accident and were being brought up to survive in a post-holocaust world. They are a new kind of human being, he explains, one with a chance of survival in the conditions that would exist "when the time comes"; radiation in the atmosphere will open the cliff door to release the children into their new world.
Appalled at what Bernard is doing, Freya refuses to join him in the project and he shoots her because of what she now knows. Offshore, a helicopter circles the yacht bearing Wells and Joan, waiting upon their inevitable deaths from radiation poisoning, as the children's faint cries for help go unheeded by an unsuspecting Weymouth.
