When British
paratroopers disappear in mid-air during routine jumps, former air force investigator Bob Megan is brought in to solve the mystery. While making inquiries at a military research laboratory headed by Dr Matthews, he has several encounters with a woman called Lorna who vanishes after each meeting. One of the troopers is found barely alive and dies on arrival at the laboratory. An autopsy performed by Dr Julie Slade reveals that the man's
biochemistry had been altered. Megan learns that all of the missing troopers had received
spaceflight training, leading Matthews to theorise that they are being adapted to survive in a non-Earth environment. Travelling to Matthews' cottage, Slade discovers that the human Matthews has been killed and his form assumed by an alien called Marthus. When Megan arrives, Marthus explains that the troopers were abducted as part of a plan to re-populate his home planet, Mygon, which has been devastated by plague. Marthus attempts to kill Megan and Slade but is incapacitated by Lorna, who is revealed to be his alien companion. Lorna shows Megan the surviving troopers, who are being held in
suspended animation, as well as her and Marthus' spacecraft. Megan asks Lorna to return the troopers in exchange for his promise to find a group of volunteers to assist her with the re-population. Lorna agrees and vanishes once again, taking Marthus and the spacecraft with her. ==Cast==