An ambulance arrives at a cottage hospital, carrying a little girl severely injured in a motorway pile-up. Dr. Mike Vernon, the duty officer, pronounces her dead on arrival. Meanwhile the radar operator of a nearby army battalion registers a slight but inexplicable blip on his screen, which he points out to his commanding officer, Major Muncaster. At the same time, Lawrence Blackburn and his mistress Barbara, travelling home from a party, are caught in a sudden strange mist and run over a young man with oriental features wearing a costume of unfamiliar material. Two similarly attired women watch unseen close by.

Police Sergeant Draycott, on a routine patrol, encounters the soliders, and he and the Major find unexplained scorch marks on the nearby heath. They proceed to the hospital to see if the staff have recently noticed any unusual occurrences. There Draycott encounters a distraught Blackburn, who attempts to convince the sceptical police officer that the accident was due to the unexplained fog. The Major backs the motorist's story and the still-suspicious Sergeant returns to his car.

The young man has an apparently non-human physiognomy and so Mike summons his off-duty colleague Clare Harland. She meets the young man who, after touching the ward sister, is now able to speak. He explains that he is from the planet Lysistrata and on the run from two police officers who were escorting him to a prison colony for a crime that he did not commit. When their spacecraft malfunctioned he fled in the escape craft. He seems surprised that Mike gives orders to the matron.

Mike reports the stranger's story to the house surgeon, Brian Carter. Since the telephones in the hospital are all out of order, Carter attempts to drive to London to inform the authorities, but is killed when his car collides with an invisible force-field. Mike, Claire and the Major demand an explanation from the young man, and are told that the Lysistratan authorities have placed a force field around the hospital.

The two female aliens have already followed Blackburn's car back to his villa in order to trace the prisoner via a communicator that was dropped on the rear seat. Their sudden appearance causes Blackburn to suffer a heart attack. Barbara has already fled the vicinity and hitches a ride with a lorry. The driver tells her that the roads have recently been in chaos due to a motorway pile-up.

In the hospital, the force field raises temperatures in the building to unbearable levels. Discovering that the water supply is unaffected, Mike attempts to seek help via the hospital's drainage system. On his return, Nurse Lim has been rendered unconscious by the Lystratan's leader, who has stolen her nurse's uniform. She apologises to Mike for the deaths caused by the tragic situation and begs the doctor to surrender the young man who, far from being a benign victim of injustice, is in fact a dangerously immature criminal. As Mike and the Lysistratan argue, the young man stabs the Major and makes a getaway, with Clare as a hostage. Mike pursues him in the Major's Land Rover, but although the prisoner, having abandoned Clare after finding himself up against troops, manages to leave the heath in the escape module, the prison ship intercepts his craft and destroys it. Clare and Mike return to the hospital, disillusioned.