For 57 years, Ellen Ripley has been in stasis in an escape shuttle after destroying her ship, the Nostromo, to escape a lethal alien creature that slaughtered her crew. She is rescued and debriefed by her employers at the Weyland-Yutani Corporation, but they are skeptical of her claims that her crew found alien eggs in a derelict ship on the exomoon LV-426, as it is now the site of the terraforming colony Hadleys Hope.
After contact is lost with the colony, Weyland-Yutani representative Carter Burke and Colonial Marine Lieutenant Gorman ask Ripley to accompany them to investigate. Still traumatized from her alien encounter, she agrees on the condition that they exterminate the creatures. Aboard the spaceship USS Sulaco, she is introduced to the Colonial Marines and the android Bishop.
A dropship delivers the expedition to the surface of LV-426, where they find the colony deserted. Inside, they find makeshift barricades and battle signs, but no bodies; two live facehuggers in containment tanks; and a traumatized young girl nicknamed Newt, the sole survivor. The crew locates the colonists grouped beneath the fusion-powered atmosphere processing station. They head to the location, descending into corridors covered in alien secretions.
At the center of the station, the marines find the colonists cocooned, serving as incubators for the creatures’ offspring. The marines kill an infant alien after it bursts from a colonist’s chest, rousing multiple adult aliens who ambush the marines, killing or capturing many of them. When the inexperienced Gorman panics, Ripley assumes command, taking control of their armored personnel carrier, and rams the nest to rescue Corporal Hicks, and Privates Hudson and Vasquez. Hicks orders the dropship to recover the survivors, but a stowaway alien kills the pilots, causing it to crash into the station. The remaining group barricades themselves inside the colony.
Ripley discovers that Burke had ordered the colonists to investigate the derelict spaceship containing the alien eggs, intending to become wealthy by recovering alien specimens for use as biological weapons. She threatens to expose him, but Bishop informs the group that the dropship crash damaged the power plant cooling system, and it will soon explode and destroy the colony. He volunteers to crawl through extensive piping conduits to reach the colony’s transmitter and remotely pilot the Sulaco’s remaining dropship to the surface.
Ripley and Newt fall asleep in the medical laboratory, awakening to find themselves locked in the room with the two facehuggers, which have been released from their tanks. Ripley triggers a fire alarm to alert the marines, who rescue them and kill the creatures. Ripley accuses Burke of releasing the facehuggers so that they would impregnate her and Newt, allowing him to smuggle the embryos past Earth’s quarantine, and of planning to kill the rest of the marines so that no one could contradict his version of events. The power is suddenly cut, and aliens assault through the ceiling. In the ensuing firefight, Burke flees but is cornered by an alien and killed, while Hudson is captured after covering the others’ retreat. Gorman and the injured Vasquez sacrifice themselves to stall the aliens; Hicks is injured, and Newt is captured.
Ripley and Hicks reach Bishop in the second dropship, but Ripley refuses to abandon Newt. The group travels to the processing station, allowing a heavily armed Ripley to enter the hive and rescue Newt. As they escape, the two encounter the alien queen in her egg chamber. When an egg begins to open, Ripley uses her flamethrower to destroy the eggs and the queen’s ovipositor. Pursued by the enraged queen, Ripley and Newt reunite with Bishop and Hicks on the dropship. All four escape moments before the station explodes with the colony consumed by the nuclear blast.
On the Sulaco, the group is ambushed by the queen, who stowed away in the ship’s landing gear. The queen tears Bishop in half and advances on Newt, but Ripley battles the creature using an exosuit cargo-loader and expels it through an airlock into space. Ripley, Newt, Hicks, and the critically damaged Bishop enter hypersleep for their return trip to Earth.