Following her explosive confrontation with a vicious Alien lifeform from the exomoon LV-426 Ellen Ripley is rescued after drifting through space in stasis for 57 years. Her employees at Weyland-Yutani Corporation disbelieve her claims that the alien killed everyone onboard her ship, the USCSS Nostromo. Years later LV-426, is colonised by the terraforming colony Hadleys Hope who are blissfully unaware of the danger. Predictably contact is lost with Hadleys Hope, so Weyland-Yutani representative Carter Burke and Colonial Marine Lieutenant Gorman ask Ripley to accompany Burke and a Colonial Marine unit to investigate the disturbance. Traumatized by her encounter with the Alien, Ripley initially refuses, but eventually agrees and makes Burke promise to exterminate, and not capture, the Aliens. Aboard the spaceship USS Sulaco, she is introduced to the Colonial Marines, their commanding officer Lieutenant Gorman, and the android Bishop, toward whom Ripley is initially hostile following her experience with the traitorous android Ash aboard the Nostromo.
A dropship delivers the expedition to the surface of LV-426, where they find the colony deserted. Inside, they find makeshift barricades and signs of a struggle, but no bodies; two live facehuggers in containment tanks in the medical lab; and a survivor, a traumatized young girl nicknamed Newt who used the ventilation system to evade capture or death. At the center of the station, the marines find the colonists cocooned, in the Aliens nest serving as incubators for the Aliens’ offspring. When the marines kill a newborn Alien, the Aliens ambush the marines, Ripley attempts to rescue the marines Hicks, 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. Ripley, Newt, Burke, Gorman, and the remaining marines barricade themselves inside the colony command center. Ripley discovers that the villainous Burke deliberately sent the colonists to investigate the derelict spaceship where the Nostromo crew first encountered the Alien eggs, believing he could become wealthy by recovering Alien specimens for use as biological weapons. The power plant is damaged and threatens to explode with the force of a 40-megaton thermonuclear weapon.
Ripley and Newt find themselves locked in the Medical bay with the two facehuggers, which have been released from their tanks. Ripley alerts 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 Alien embryos past Earth’s quarantine, and of planning to kill the rest of the marines. Then Aliens attack through the ceiling. Hudson, Burke, Vasquez, and Gorman are all killed in the attack and Newt is captured. Ripley and an injured Hicks reach Bishop in the second dropship, but Ripley refuses to abandon Newt so a heavily armed Ripley enters the hive to rescue Newt. As they escape, the two encounter the Alien queen in her egg chamber. Pursued by the queen, Ripley, Newt, Bishop and Hicks escape however the group discover the Alien queen has stowed away on the dropship’s landing gear and Ripley clashes with her in an exciting showdown