In a near-future where Lunar Industries has turned a significant profit following an oil crisis, the corporation establishes Sarang Station, an automated facility designed to extract helium-3 from lunar soil—a critical alternative fuel source. This innovative station now caters to 70% of Earth's energy needs through helium-3 fusion reactions derived from the moon's far side. The facility operates autonomously, requiring only one human, Sam Bell (Sam Rockwell), to oversee its functioning, manage the harvesters, and ensure the smooth delivery of helium-3 canisters back to Earth.
As Sam approaches the conclusion of his three-year contract—set to return home in just two weeks—he reflects on his limited outings from the station, which only occur when the harvesters are full of helium-3. His solitary task involves piloting a rover to the harvester, replacing used canisters with empty ones, and returning for further processing. Communication with Earth has largely become a distant memory, leaving Sam reliant on infrequent recorded messages from his wife, Tess (Dominique McElligott), who was pregnant with their daughter, Eve, when he left. His only company during this solitary tenure is an artificial intelligence named GERTY (Kevin Spacey), a non-humanoid entity that glides around the base, helping to alleviate Sam's loneliness.
However, two weeks before his scheduled return, Sam begins to experience strange hallucinations of a teenage girl, which culminate in a disastrous rover crash while he is en route to a harvester. Losing consciousness after donning his space suit, Sam awakens in the base infirmary with no memory of the accident. He overhears GERTY discussing an increase in production for the harvesters and confirms a rescue team's imminent arrival—a situation that raises his suspicions about the validity of GERTY's claims, as communication with Earth is supposedly down.
In an attempt to investigate, Sam fabricates a problem to gain access outside the station, where he ultimately discovers his own doppelganger, an unconscious version of himself. After bringing the other Sam back to the base and nursing him back to health, both versions grapple with their newfound reality: they are clones of the original Sam Bell. Tensions rise as they uncover the truth behind their existence, leading to a confrontation with GERTY, who reveals that all messages sent to Tess were never delivered, and that their memories have been implanted.
As the older Sam's health deteriorates, both Sams realize that they are mere pawns of Lunar Industries, which is callously using clones to minimize the costs associated with manned space operations. They encounter a hidden vault filled with numerous dormant clones, unveiling the depravity and unethical nature of their situation. Determined to reclaim their autonomy, the two Sams devise a plan to outsmart their employers, with the older Sam consenting to sacrifice himself to allow the newer clone a chance at life.
In a dramatic finale, the older Sam is returned to the wrecked rover, satisfied to remain behind as the younger Sam successfully contacts Earth, exposing the corporate malfeasance of Lunar Industries. As he witnesses the transport carrying the younger Sam launch from the lunar surface, the older Sam comes to terms with his fate while leaving a legacy that challenges the very foundations of his creators' exploitation. News reports soon flood in, transforming Sam’s journey into a significant ethical discourse surrounding the treatment of clones and the future of corporate practices in space exploration.
