Decades-old footage from NASA's abandoned Apollo 18 mission uncovers the mysterious reasons behind the United States' absence from lunar exploration since 1973. Initially dismissed, the mission is suddenly approved as a top-secret Department of Defense operation. Commander Nathan Walker (Lloyd Owen), Lieutenant Colonel John Grey (Ryan Robbins), and Captain Benjamin Anderson (Warren Christie) are launched into space to deploy detectors aimed to warn against impending ICBM threats from the USSR.
While Grey remains in orbit aboard the Freedom Command/Service module, Walker and Anderson land on the Moon using the lunar module Liberty. As they attempt to install one of their detectors, they also collect samples of moon rocks. However, sleep eludes them as strange noises fill the atmosphere, leading to a startling incident where a camera captures a small rock moving nearby. Houston, represented by the voice of Andrew Airlie, assures them that the disturbances are merely interference from the ICBM detectors. But things take a bizarre turn when Anderson finds a moon rock sample on the floor of the Liberty, despite their earlier precautions to secure them.
Their exploration uncovers eerie footprints, guiding them to a Soviet LK lander that is functional yet eerily blood-stained. Anderson ventures into a dark crater and uncovers a deceased cosmonaut, but when Walker raises concerns about the Soviet presence, Houston brushes off his queries, instructing them to continue their mission.
The following day, they discover that their American flag has vanished. As they prepare to conclude their tasks and depart the Moon, violent tremors shake their lunar module, forcing them to abort the launch. An inspection points to significant damage and non-human tracks; Walker is convinced of extraterrestrial involvement when he feels something moving inside his spacesuit—an unnerving encounter with a spider-like creature.
Walker soon goes missing, only to be found unconscious. Upon an examination, a mysterious wound on his chest reveals a moon rock embedded within. As they struggle to maintain contact with Houston and Grey, interference from an unknown source complicates their situation. A paranoid Walker, displaying signs of infection from his wound, becomes increasingly delusional and attempts to sabotage the cameras in Liberty. His reckless actions lead to a catastrophic depressurization event.
Faced with a diminishing oxygen supply, the duo traverses to the Soviet LK lander, yet Walker's paranoia causes him to flee, fearing he could spread the infection to Earth. Before long, Anderson crashes while trying to compel Walker to return, later waking to find Walker entrapped and pulled into the crater by moon creatures. In a desperate move, Anderson uses a strobe light for illumination, only to realize that the “moon rocks” are not what they seem.
Making contact with USSR Mission Control, Anderson learns the grim truth from the deputy secretary of the Department of Defense—returning to Earth is not an option, given the risk of contamination. When Anderson attempts to launch the LK lander to reunite with Grey, he is assaulted by Walker, now fully overtaken by the alien creatures. In a chaotic climax, the lander's reduced gravity causes rocks to float, revealing that some harbored alien life forms, leading to Anderson's own infection.
As the dramatic tension unfolds, Grey warns Anderson of an impending collision, and just as the footage reveals their fate, it abruptly cuts to earlier moments of the astronauts enjoying a barbecue with loved ones. The official narrative concludes with false reports of the astronauts' deaths in accidents that left their bodies unrecoverable, while an epilogue raises questions about the fate of moon rocks brought back from earlier Apollo missions, suggesting a cover-up of the horrific truth that lay hidden on the lunar surface.
