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Scientists Jack Costain, Ann Barlow, and Professor Morley visit the landing site of a UFO whose course they have traced by radar but find only a 6-inch sphere, which they take to their research laboratory. Ann, working late one night, encounters a scaly-clawed creature. The major in charge of the Army unit guarding the sphere disbelieves her story, but Jack finds a giant footprint leading from the window of the storeroom. Morley speculates that the sphere is a receiver for the transmission of matter from another planet but dies as he attempts to observe its activation. The alien disappears with the sphere, and the major is killed shortly afterwards. In the weeks that follow, several young women are reported missing. Superintendent Hartley of Scotland Yard learns that all of them answered a classified advertisement soliciting models and contacted "Medra" at a Soho bookshop. Ann answers the advertisement and discovers that Medra is the alien being, a creature half-human and half-beast who has come from Jupiter's third moon, Ganymede. Jack and Hartley break into the bookshop and find Ann strangled. Following the disappearance of another woman, they trace Medra to a farm. He explains that he came to Earth to gather women for genetic experiments to benefit Ganymede's population, a race of mutants descended from survivors of atomic warfare long ago. His mission complete, he enters the sphere and returns to Ganymede.