In a voice-over narration, set in the late 22nd Century, we follow the United Planet cruiser C57D, which is a year into its journey towards Altair on a special mission. Commander J.J Adams (Leslie Nielsen) instructs the crew to prepare for a deceleration as they transition from light speed into normal space. With the ship now ready, Adams directs pilot Jerry Farman (Jack Kelly) to chart a course for the planet's fourth orbit.
Upon arrival, the crew learns that they are investigating the fate of the Bellerophon expedition, lost two decades prior. As they circle the planet searching for survivors, they encounter a radar signal from the surface, leading to contact with Morbius (Walter Pidgeon), who cryptically suggests that he needs no help and cannot assure the safety of the ship. Confused yet intrigued, Adams recognizes Morbius as a member of the original crew but questions the hazardous warning.
Morbius, unrelenting, provides landing coordinates in a desert area where the ship descends, and there, the crew meets the robot Robby. Robby introduces himself with eagerness, displaying capabilities that astonish the visitors. During a meal at Morbius's residence, he showcases Robby’s unwavering loyalty and even attempts to command Robby to shoot Adams, who is saved from harm when Robby refuses, locking into a mechanical paralysis until the command is revoked.
As conversations unfold, Morbius reveals that he is among the last survivors of the Bellerophon crew. His wife has also perished—months after the incident—and he shares chilling tales of how many were killed by a fearsome force residing on the planet. Despite the grim fate of his crewmates, Adams is perplexed at how this danger has remained dormant during Morbius's years living in isolation. When Altaira (Anne Francis), Morbius's daughter, enters, the atmosphere shifts; Farman quickly becomes enamored and playfully engages with her, sparking a budding romance.
Altaira astounds the crew with her ability to connect with a wild tiger, further enchanting Farman. However, with preparations underway for communication with Earth, a complication arises: the ship might be rendered inoperable for up to ten days. Stricken at the thought of this, Morbius offers his robot's assistance, showcasing sophisticated technology while underlying the risks of dependency.
As Robby engages in friendly banter with the crew, Cookie (Earl Holliman) seeks out Robby’s assistance in synthesizing bourbon. Amidst light-hearted interactions, an unseen threat looms, likely instigating a series of strange incidents aboard the ship when a scream reverberates through the compound.
During the ensuing chaos, Adams encounters Altaira, which leads to a tense moment where he accidentally sees her swim, setting off feelings of jealousy. Following a roaring encounter with a disguised beast, Morbius reveals the existence of the Krell, a now-extinct advanced civilization that once thrived on the planet. His insights into their downfall paint a haunting picture of intelligence without wisdom, suggesting that a monster born from human subconscious is now unleashed from within.
As the narrative advances, Adams grapples with emerging dangers, leading to a desperate struggle for survival against manifestations of inner fears, resulting in shocking loss. Morbius finally accepts his culpability for these created horrors, realizing the peril he poses to Altaira and the rest of the crew.
In a climactic standoff, while racing against time, Morbius implores for the self-destruct sequence to be initiated to preserve the remaining lives. With heavy hearts, Adams and Altaira watch from a distance as the planet meets its fiery end, believing in the legacy of Morbius that will shine through. With Robby at the helm of their ship, they escape into the cosmos, leaving the devastation behind but carrying the weight of an enduring memory, ensuring that Morbius's spirit does not fade into darkness.
