The inhabitants of Ganymede need to find mates from another world or they will become extinct. They soon discover a suitable breeding stock amongst the females of planet Earth.

John Saxon
as Dr. Jack Costain

Patricia Haines
as Ann Barlow

Alfred Burke
as Detective Supt. Hartley

Maurice Denham
as Dr. Morley

John Carson
as The Major

Ballard Berkeley
as Cmdr. Savage

Stanley Meadows
as Det. Tom Grant

Barbara French
as Joyce Malone

Robert Crewdson
as Medra

Aubrey Morris
as Thorburn