Aliens starts with Ellen Ripley being saved after 57 years of wandering in space. In Weyland-Yutani Corporation, she is informed that her ship, Nostromo is already gone while the employees do not believe that an alien had gotten aboard of her ship that annihilated her entire crew.
In Zena Reticuli, the exomoon LV-246 now serves as home of Hadley’s Hope colony. After losing contact with Hadley Hope, Colonel Marine Lieutenant Gorman orders Ripley to go with Carter Burke in investigating what happened. Ripley has some doubts about going but her dreams about the alien push her to do the investigation. As an insurance, she makes Burke swear that he will kill any aliens that they will encounter. They get on board, USS Sulaco, where Ripley meets the crew and an android named Bishop.
They are transported to a now deserted LV-246. When they check out the place, they see signs of struggle but they find no bodies. In the medical lab, they encounter two facehuggers and survivor, Newt, who hides in the ventilation system to avoid being captured. The investigating team uses the colony’s computer to determine the location of the other colonists.
When they reach the center of the station, they find that the remaining colonists have been cocooned, harboring the aliens’ young. The marines kill one of the newborn aliens and this wakes up the rest of the terrestrial creatures, killing the soldiers and apprehending the others. Ripley grabs the wheel from Gorman and drives to save the other marines. A marine named Hicks calls the dropship for rescue but an alien slaughters the pilots which causes the dropship to crash. The rest of survivors then retreat inside the colony to protect themselves.
Once inside, Ripley finds out the real reason why Burke sent the colonists to LV-246. She learns that Burke intends to recover any alien remains that can be potentially used as biological weapons. As Ripley threatens to expose his plans, Bishop announces an imminent problem. The power plant has been damaged during the attack and they only have little time left before the 40-megaton thermonuclear weapon explodes. Bishop offers to go through the pipes in order to reach the colony’s transmitter and from there, remotely maneuver a dropship that they can use.
After some time, Ripley and Newt fall asleep only to discover that they are locked up in the lab with two facehuggers. Ripley manages to set off the alarm and the marines come to their rescue. Ripley accuses Burke of locking them up in the laboratory. She argues that Burke want the facehuggers to impregnate them so that he can smuggle the aliens in Earth. Before the marines can take any action against Burke, the power goes off and a group of aliens start attacking them. The rest of the crew are killed off one by one as Hudson, Burke, and Newt are held hostages.
Ripley and Hicks escape. They find their way to the second dropship. The two men are ready to escape but Ripley does not want to leave Newt behind. She looks for some kind of weapon and heads back to the colony to save Newt. As they escape, they come across the alien queen in her egg chamber. When Ripley destroys the alien eggs, the queen becomes enraged and it chases Ripley and Newt as they find their way back to Bishop and Hicks. Minutes before the thermonuclear weapon explodes, the remaining survivor escape using the spare dropship.
Once inside Sulaco, their relief is broken when the alien queen makes a surprise attack on them. The alien badly injures Bishop while Ripley forces it outside the ship using a cargo-loader. The movie ends up with the group entering a hypersleep mode before their vessel heads back to Earth.