In the distant future, an overcrowded Earth relies on research conducted by scientists in remote stations across the solar system. One of these, a small experimental hydroponics research station on Saturn's third moon, is run solely by Adam (Douglas) and his colleague and lover Alex (Fawcett). Adam, the younger Alex and their dog Sally enjoy their isolation, far from an overcrowded and troubled Earth. The couple have been on Saturn 3 for three years, but Alex has spent all her life in space.
Alex and Adam's idyll is broken by the arrival of Captain Benson (Keitel), who has been sent to Saturn 3 as part of a project to replace at least one of the moon's scientists with a robot. Benson assembles the robot and names him Hector. One of the first of its kind, a "Demigod Series", Hector relies on "pure brain tissue" extracted from human fetuses and programmed using a direct link to Benson's brain. Benson states that it will render one of the crew "obsolete", most likely Major Adam who tells his partner that he is close to "abort time" according to the government on Earth.
Unknown to both Alex and Adam, Benson is a homicidal sociopath who murdered the captain originally assigned to Saturn 3 and took his place on the mission. Benson had actually failed a crucial test of psychological stability.
As Benson uses the link to his brain to program the robot, Hector acquires Benson's homicidal nature and his lust for Alex. Hector initially kills Sally the dog and then assaults Benson, having learned about Benson being a murderer during the reprogramming process. Adam and Alex manage to disable the robot while it is recharging.
Believing the danger over, Adam accuses Benson of gross incompetence and orders him to dismantle the robot and return to Earth when an eclipse ends. Benson obliges, but Hector is soon reassembled by the base's older robots. Resuscitated, Hector murders Benson while he is dragging Alex to his ship. Hector then blows up Benson's spacecraft before Adam and Alex can escape in it, trapping them all on Saturn 3 together.
Overpowering the humans, Hector installs a brain link at the top of Adam's spine which will give the robot direct access to Adam's brain. Before Hector can make the connection, Adam destroys it, sacrificing himself by detonating explosives hidden on his person.
In the final scene, Alex, now alone, is shown aboard an Earth-bound spacecraft.
Who is shown aboard an Earth-bound spacecraft?
Alex