In the 23rd century, after more than a year's journey, the United Planets starship C-57D arrives at the distant planet Altair IV to determine the fate of the ship Bellerophon, sent there 20 years before. Dr. Edward Morbius, one of the original expedition's scientists, warns the ship not to land for safety reasons, but Commander John J. Adams ignores his warning.
Adams and Lieutenants Jerry Farman and "Doc" Ostrow are met by Robby the Robot, who transports them to Morbius' residence. Morbius describes how all other members of their expedition had been killed, one by one, by an unseen "planetary force", with the Bellerophon being vaporized as the last survivors tried to escape. Only Morbius, his wife (who Morbius claims later died of natural causes), and their daughter Altaira were somehow immune. Morbius offers to help the starship return home, but Adams says he must receive further instructions from Earth.
Leslie Nielsen and Anne Francis in Forbidden Planet
The next day, Adams finds Farman kissing Altaira. Furious, he rebukes Farman and criticizes Altaira for wearing revealing clothing. That night, an invisible intruder sabotages communications equipment aboard the starship. The next morning, Adams and Ostrow go to Morbius' residence to discuss the intrusion. While waiting, Adams happens upon Altaira swimming. After she dons a new, less revealing dress, Adams apologizes for his behavior toward her, and they kiss. They are suddenly attacked by Altaira's pet tiger, and Adams is forced to disintegrate it with his blaster.
Morbius appears and tells Adams and Ostrow that he has been studying artifacts of the Krell, a highly advanced race that mysteriously perished in a single night 200,000 years before. One such device enhances the intellect, which Morbius had used. He barely survived, but his intellectual capacity had doubled. Another is a vast 8,000-cubic-mile (33,000 km3) underground machine, still functioning, powered by 9,200 thermonuclear reactors. Adams tells Morbius he must share these discoveries with Earth, but Morbius refuses, saying, "Humanity is not yet ready to receive such limitless power."
Adams erects a force field fence around the starship, but the unseen intruder easily passes through and brutally murders Chief Engineer Quinn, who was repairing the damaged communications equipment. Morbius warns Adams of his premonition of further deadly attacks. That night, the intruder is detected approaching. Its outline and features become visible when it enters the force field and blasters are fired at it, to little effect. The thing kills Farman and two other crewmen. When Morbius is awakened by Altaira's screams, the creature suddenly vanishes.
Adams tries to persuade Altaira to leave. Ostrow sneaks away and uses the Krell intellect enhancer but is fatally injured. Before dying, he informs Adams that the underground machine's purpose was to create anything by mere thought, anywhere on the planet. However, he tells Adams the Krell forgot one thing: "Monsters from the id." The machine gave the Krell's own subconscious desires free rein with unlimited power, causing their own extinction. Adams deduces that Morbius's subconscious created the thing that both killed the original expedition members and attacked his crewmen; Morbius refuses to believe him.
Altaira tells Morbius that she is leaving Altair IV with Adams. Robby detects the creature approaching; Morbius commands Robby to kill it, but the robot knows it is Morbius and shuts down, being programmed to never kill a human. Adams, Altaira, and Morbius hide in the Krell lab, but the creature melts its way through the thick doors. Morbius finally accepts the truth and confronts and disowns his other self, but is fatally injured by the creature as it vanishes. Before he dies, he has Adams activate a planetary self-destruct system, warning them to be far away in deep space. At a safe distance, Adams, Altaira, Robby, and the surviving crew witness the obliteration of Altair IV. Adams reassures Altaira that in about a million years, the human race will stand where the Krell did. They embrace as C-57D heads back to Earth.
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