The film begins with a commercial from Lunar Industries. In the late 21st Century, Earth has had an energy crisis. Most coal, oil and natural gas reserves are nearly gone. Our new primary source of energy is Helium-3, which is found on the moon. Lunar Industries is responsible for the mining of Helium-3 and sending it back to Earth. We are then shown Lunar Industries Sarang Mining Base on the moon.
There is only one person in the crew, and his contract is for three years. We are introduced to the employee, Sam Bell (Sam Rockwell). He has grown out his hair and has grown a beard during his time on the base. He puts on a spacesuit and goes outside, where he drives a land rover over to a Helium-3 harvester. He collects the Helium-3 and then sends it back to Earth using a rocket. We see how lonely Sam is when he continually talks to himself.
Sam has video messages from his bosses and family, but he can't communicate with them via live feed, due to a satellite being jammed. Sam's only companion is GERTY (voice by Kevin Spacey), an intelligent robot who controls the base and looks after Sam. As GERTY gives Sam a haircut, he asks him if everything's okay. He noticed that he's been acting unusual lately. Sam says that he's fine and then asks if anyone has fixed the satellite yet.
GERTY informs him that its a low priority of the company. Even though he has two weeks left in his contract and will return to Earth soon, he thinks that it's fair for the satellite to be fixed for the person who will replace him. He also says that he has a headache. Afterward, he looks at an old video message sent by his wife, Tess (Dominque McElligott). She knows that he's lonely on the Moon's surface all by himself, but she's proud of what he's doing.
She has their little daughter Eve say "astronaut." Sam passes the time by working on a miniature model of his hometown and watering his plants in the greenhouse. Sam watches an old TV show and then turns the TV off. As Sam fills his glass with hot water, he hallucinates, seeing a woman in his seat. Distracted, Sam's hand gets burned by the hot water. GERTY bandages Sam's hand and asks him what happened.
Sam says that he was distracted by the TV, but GERTY knows that he's lying since the TV was off when he came into the room. He says that maybe Sam is seeing things. Sam goes to sleep and dreams of being home with Tess. The next day, Sam wakes up and runs on a treadmill. He experiences some chest pains, but thinks nothing of it. GERTY makes Sam breakfast and cheers him up by reminding him that he has two weeks left. Sam is excited about going home.
Sam puts on his spacesuit and uses the rover to go collect more Helium-3. On the drive though, Sam hallucinates about seeing the woman again and crashes the rover into a harvester. The atmosphere is compromised, so Sam puts on his helmet. Sam wakes up in the infirmary. He looks fine but is too weak to move. GERTY has been monitoring his health and he asks Sam if he remembers anything from the crash. Sam doesn't, and GERTY tells him to go back to sleep.
Sometime later, Sam starts to get frustrated about being kept in the infirmary since he feels better. GERTY says that he might have suffered from brain trauma and continues to conduct tests to make sure he's all right. Later on, Sam (healthy) gets up and walks around the base. He overhears GERTY talking to Lunar Industries via live feed. GERTY, realizing that Sam is nearby, shuts down the transmission. Sam asks him what's going on.
GERTY says that he has informed Lunar Industries about the crash. They will be sending a rescue team to fix the harvester and then they will send Sam back to Earth. Sam says that he can fix the harvester himself and wants to go outside. GERTY has been given orders to keep Sam inside the base until the rescue team arrives. Sam also confronts GERTY about talking live to the company, but GERTY says that Sam is seeing things.
Sam feels like he's being treated like a child and grabs some tools. He opens a vent and cuts into a tube, causing a leak of gas to spray out. GERTY comes over and works on fixing it. Sam says that some meteorites might have caused the problem. He says that there might be a breach in the exterior shell of the base and that he should go inspect it. After some arguing, Sam tricks GERTY into letting him outside.
Sam finds that his spacesuit is missing, and so he takes a spare one. He gets into another rover and drives out to the harvester where the crash occurred. The first rover is still there. Sam opens the hatch and finds a body inside. Upon further inspection, he sees that the body looks just like him but with bruises on his face. Sam brings the body back to the base. He asks GERTY what's going on and why the man looks like him.
GERTY states that the man is Sam Bell. (To make it clear, I'll refer to the bruised man as Sam 1 and the healthy man as Sam 2) Sam 1 wakes up in the infirmary and we see that his hand is bandaged (making him the one we were introduced to before the crash). Sam 1 sees Sam 2 standing in the room. He asks GERTY who the other man is, but GERTY has Sam 1 go back to sleep. Later on, Sam 1 is well enough to walk around the base.
There's some tension between the two Sams, with Sam 2 being the more aggressive one. Sam 1 tries talking to Sam 2, but he's more focused on working out. Sam 2 knows that they are clones of the real Sam Bell, which Sam 1 refuses to believe. Sam 1 asks GERTY if they are clones, but GERTY changes the subject. Sam 1 wants to shake Sam 2's hand since he's lonely and wants some human contact. Sam 2 snubs him, saying maybe later.
Sam 1 tries to convince Sam 2 to relax by playing against each other in ping pong. Sam 1 easily beats Sam 2, causing him to lose his temper. Sam 1 works on his miniature model of his hometown. Sam 2 asks him how long he's been working on it. Sam 1 says that the model was already started before he found it, and so he's been adding new buildings and people.
Sam 2 takes this as further proof that they are clones, since someone else started the model and they share the same memories of the hometown. He says that there must be a hidden place somewhere on the base where Lunar Industries keeps the cloned bodies. Sam 1 thinks Sam 2 is crazy. Sam 2 starts to go throughout the base, searching for a secret room. He searches everywhere except for where the model is.