In 2035, Lunar Industries have made a fortune after an oil crisis by building a large lunar mining base on the Moon, called Sarang, and since then have been extracting a large amount of helium-3 back to Earth. The system is practically automated, but the company requires one person on the station to monitor and send back shipments of Helium-3 when needed. Consequently, Sam Bell is that sole employee who is serving out his 3 year contract on the moon. To save costs, workers stay up there for long periods in solitary confinement. With not another living soul on the moon, Sam occupies himself by talking to his plants, carving a miniature model of his town, and talking to computer with an emoticon personality named Gherty 3000. All he wants is to return to Earth to be with his wife Tess and their infant daughter Eve, who was born just prior to his leaving for this job.
After a freak accident at the station nearly leaves him dead, Sam wakes up to be greeted by … himself. The “new” Sam Bell, to be sure, is as befuddled by this meeting as his counterpart. Having just been awakened that morning for what he assumed to be the start of his own three-year stint, he didn’t expect to rescue an identical twin he never knew existed. Eventually, the two Sams discover the terrible truth: they are in actuality clones, created by Lunar Industries for cheap labor.
Though not explicitly defined in the movie, according to director Duncan Jones, the clones were designed to have a three-year lifespan. Three years was estimated as the longest amount of time each clone would continue working without losing motivation to do their job well, in Jones’ words, giving Lunar industries “the most bang for their buck.”