In the distant future, the gargantuan blue humanoid Draags have brought human beings (who are called Oms as a play on the French word for "man", Homme) from Earth to the planet Ygam, where they maintain a technologically and spiritually advanced society. They consider them animals, and while some Oms are kept as pets by Draags, others live in the strange wilderness and are periodically slaughtered by the Draags who wish to keep their population controlled. Draags have much longer lifespans than Oms, but reproduce much less.
In 2069, the unmanned Sun Probe locates a planet lying on the same orbital path as Earth on the opposite side of the Sun. Dr Kurt Hassler (Herbert Lom) of the European Space Exploration Council (EUROSEC) has been relaying the spacecraft's flight data to a rival power in the East; after tracing the transmissions to Hassler's laboratory, Security Chief Mark Neuman (George Sewell) catches the scientist in the act and kills him. EUROSEC director Jason Webb (Patrick Wymark) convinces NASA representative David Poulson (Ed Bishop) that the West must be the first to send a mission to investigate the planet. With EUROSEC member states France and Germany unwilling to provide financial support, Webb obtains majority funding from NASA; American astronaut Colonel Glenn Ross (Roy Thinnes) and British astrophysicist Dr John Kane (Ian Hendry), the head of the Sun Probe project, are assigned to the mission.
In the 23rd century, to discover the fate of an expedition sent 20 years earlier, starship C-57D reaches the distant planet Altair IV. Dr. Edward Morbius (Walter Pidgeon), one of the expedition's scientists, contacts the starship. He states no assistance is required, warning the Earth ship away, saying he cannot guarantee their safety. Commander John Adams (Leslie Nielsen) insists on landing.