The spaceship Nostromo is returning to our home-planet, Earth. Nostromo had recently visited Thedus which the crew collected 20,000,000 tons of mineral ore. During the flight back, the crew are in statis. Nostromo recieves a distress signal from a unknown planet which awakens the crew from their deep sleep. The crew then detaches the Nostromo from the refinery and lands on the nearby planet which damages the ship. Captain Dallas and a few others go to investigate what had caused the signal to be released while Ripley and the rest stay behind to stand guard of the ship.
Dallas and the rest discover that the signal is coming from a nearby spaceship, they proceed to investigate. Inside the alien space-craft, they find a large alien whose ribs had been ripped open from the inside. Meanwhile, Ripley and the others determine that the signal that had been sent out was a type of warning, shortly after Kane discovers numerous alien eggs which then one of them attach to his face (facehugger). Dallas and Lambert assist Kane (unconscious) back to Nostromo. They try to remove the face-hugger from Kane's face, but fail - then finding out that it's blood is very corrosive and can easily harm.
Kane awakens mysteriously fine and well but when he is eating a meal (before going back into stasis) he starts to choke until we see the first chest burster making an appearance, which kills him and escapes into the depths of the ship. Since the crew lacked weapons, they tried to hunt it down by using motion trackers and electric pods. Dallas follows the crew's cat into a room where a fully grown Xenomorph attacks him and deprives of his body. This now puts Ripley in charge of the crew. Ripley then explains the ship can only support 4 members.
Ripley then access the ship's computer system and discovers that Ash has been ordered to return the Xenomorph to Nostromo's employers even though it could cost the crew's lives. Ash attacks Ripley but Parker stops him and burns Ash, in which Parker realizes Ash is an android. Although, before Ash gets burned, he predicts that the crew will not survive long enough to reach back home. The last three crew members plan to activate the self destruct mechanism and escape the shuttle, returning to Earth. This does not work out since Parker and Lambert are killed by the Xenomorph whilst collecting the supplies. Ripley, being the last person alive, atempts to abort the self destruct mechanism but she is unsucessful. She returns and to find that the Xenomorph is gone which she then escapes as the ship explodes.
As she preapres to once again enter stasis, she discovers that the Xenomorph is hidden within the shuttle. She equips her spacesuit and opens the hatch and causes a massive explosion causing the Xenomorph to be forced into the open doorway. She shoots it with her remaining weapon; the grapling gun which launches it out but the gun is then forced out of her hand and gets ledged in the closed doorway releasing the Xenomorph into the shuttle. The Xenomorph attempts to crawl into one of the shuttle's engines but Ripley gets to the engine just in time and launches the Xenomorph into outer space. She then retrieves the cat and puts herself into stasis for the return trip to Earth.