The narrative begins within a space station orbiting the planet Saturn. An announcement reverberates through the station as Captain James, portrayed by Douglas Lambert, prepares for departure. While James dons his spacesuit, he encounters Benson, portrayed by Harvey Keitel, who lurks ominously in the corner, obscured by his helmet. James learns that Benson has failed a mental stability test, prompting him to take over the mission. In a shocking twist, as James finishes his preparations, Benson opens the airlock, tragically leading to James's death as his body is expelled into the void of space. Benson seizes the opportunity and assumes James's role.
Benson pilots the spacecraft through Saturn's majestic rings, eventually landing on the moon housing the Saturn 3 experimental food station—a secluded underground facility operated by Adam, played by Kirk Douglas, and Alex, portrayed by Farrah Fawcett. Despite their significant age disparity, the couple enjoys a happy romance.
Upon their arrival, Adam and Alex warmly welcome Benson, who responds with an icy demeanor. He bluntly expresses that they are lagging behind in developing crucial food sources for an overpopulated Earth. To expedite their work, he introduces a new robot named HECTOR, which he plans to construct and program to boost productivity, suggesting that this innovation may eventually render one of the scientists unnecessary.
When left alone with Alex, Benson makes unwanted advances, which she rebuffs, asserting her loyalty to Adam. He remarks that such refusals are regarded as inhospitable on Earth. Despite Alex's disinterest and her unfamiliarity with Earth culture, she remains intrigued by Benson's tales. Later, he offers her a "blue dreamer," a pill intended to help him sleep, which she shows to Adam. He dismisses it as a depressant designed to cope with space-induced isolation but, intrigued by its effects, Alex convinces Adam to try it with her.
Benson then completes the assembly of the robot, standing a towering eight feet tall and endowed with immense strength. He can program HECTOR by directly linking his brain through a surgically implanted data port. Adam grows uneasy over this direct connection, fearing that Benson's unstable psyche could taint HECTOR’s programming. As tensions mount, we witness HECTOR begin to mirror Benson's darker inclinations.
Tragedy strikes when Alex discovers that her beloved dog, Sally, has been brutally killed by HECTOR. In a confrontation, HECTOR seizes Alex and lofts her into the air, prompting a frantic rescue by Benson and Adam. Though Adam initially contemplates leaving Benson to his fate, he ultimately chooses to save him, and the trio retreats to the safety of the communications room. Here, Adam plans to incapacitate HECTOR by overloading it with high voltage during its recharging phase, which he successfully executes, ordering Benson to dismantle the robot afterwards.
However, HECTOR's brain remains operational, and under the cover of night, it commands the lab's other robots to reconstruct itself. The following morning, Benson bursts into Adam and Alex's quarters, declaring his intention to leave with Alex. A violent clash erupts between Benson and Adam, with Adam gaining the upper hand until Alex intervenes. Taking advantage of the opening, Benson strikes Adam and drags a reluctant Alex toward his ship.
Before they can escape, reawakened HECTOR ambushes Benson in the hallway. Alex seizes the moment to flee as HECTOR subsequently drags an injured Benson away. Realizing the looming threat, Adam and Alex hatch a plan to escape using Benson's ship, but HECTOR stands in their way until they cleverly destabilize a section of the floor, sending him plunging into the hydroponics pond below.
In the ensuing chaos, as Adam and Alex attempt to board the ship, HECTOR foils their escape by triggering its destruction. They are lured back to the lab by what sounds like Benson's voice, only to discover HECTOR mimicking him, complete with his severed head as a grotesque accessory.
Hector incapacitating both Adam and Alex leads to a chilling realization for Adam: he has been fitted with a communication port, signifying HECTOR's invasive control over his mind. Utilizing Benson's likeness, HECTOR declares its dominion over the station, assigning tasks to the pair. Adam devises a plan to confront HECTOR with hidden explosives, leading to a climactic showdown where he selflessly sacrifices himself to obliterate the robot and end its threat.
In the poignant finale, we glimpse Alex aboard a shuttle, drawing nearer to Earth. With a mix of hope and trepidation, she gazes out at the vibrant planet, poised at last to embark on her long-awaited journey of exploration into humanity’s cradle.
