Ellen Ripley has been in stasis for 57 years aboard an escape shuttle after destroying her ship, the Nostromo, to escape an alien creature that slaughtered the rest of the crew.[a] She is rescued and debriefed by her employers at the Weyland-Yutani Corporation, who are skeptical about her claim of alien eggs in a derelict ship on the exomoon LV-426,[b] since it is now the site of a terraforming colony.
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 by her alien encounter, she agrees on the condition that they exterminate the creatures. Ripley is introduced to the Colonial Marines on the spaceship Sulaco but is distrustful of their android, Bishop, because the android aboard the Nostromo betrayed its crew to protect the alien on company orders.
A dropship delivers the expedition to the surface of LV-426, where they find the battle-ravaged colony and two live alien facehuggers in containment tanks, but no bodies or colonists except for a traumatized young girl nicknamed Newt. The team locates the colonists beneath the fusion-powered atmosphere processing station and heads to their location, descending into corridors covered in alien secretions. At the station center, the Marines find opened eggs and dead facehuggers alongside the cocooned colonists now serving as incubators for the creatures' offspring. The Marines kill an infant alien after it bursts from a colonist's chest, rousing several adult aliens who ambush the Marines and kill or capture many of them. When the inexperienced Gorman panics, Ripley assumes command, takes control of their armored personnel carrier, and rams the nest to rescue Corporal Dwayne Hicks and Privates Hudson and Vasquez. Hicks orders the dropship to recover the survivors, but a stowaway alien kills the pilots, and it crashes into the station. Almost out of ammunition and resources, the survivors barricade themselves inside the colony.
Ripley discovers that Burke ordered the colonists to investigate the derelict spaceship containing the alien eggs, intending to profit by recovering them for biological weapon research. Before she can expose him, Bishop informs the group that the dropship crash damaged the power-plant cooling system and the plant will soon overheat and explode, destroying the colony. He volunteers to travel to the colony transmitter and remotely pilot the Sulaco's remaining dropship to the surface.
After falling asleep in the medical laboratory, Ripley and Newt awaken to find themselves trapped with the two released facehuggers. Ripley triggers a fire alarm to alert the Marines, who rescue them and kill the creatures. She accuses Burke of releasing the facehuggers to implant her and Newt with alien embryos, allowing him to smuggle them through Earth's quarantine. The power is suddenly cut, and aliens attack through the ceiling. In the ensuing firefight, the aliens kill Burke, subdue Hudson, and injure Hicks; the cornered Gorman and Vasquez sacrifice themselves to avoid capture. Newt is separated from Ripley and taken by the creatures. Ripley brings Hicks to Bishop in the second dropship, but she refuses to abandon Newt and arms herself before descending into the processing station hive alone to rescue her. During their escape, they encounter the alien queen surrounded by dozens of eggs, and when one begins to open, Ripley uses her weapons to destroy them all and the queen's ovipositor. Pursued by the enraged queen, Ripley and Newt join Bishop and Hicks on the dropship and escape moments before the station explodes, consuming the colony in a nuclear blast.
Aboard the Sulaco, the group is ambushed by the queen, who stowed away in the dropship's landing gear. The queen tears Bishop in half and advances on Newt, but Ripley fights the creature with an exosuit cargo loader and expels it through an airlock into space while the damaged Bishop keeps Newt safe. Ripley, Newt, Hicks, and Bishop enter hypersleep for their return trip to Earth.
