A group of deep space cargo haulers bring a deadly alien species onto their ship.
In the year 2120, the crew of the commercial towing vehicle, the Nostromo registration, 180246 are returning to Earth with a cargo of mineral ore. While in hibernation floating through the Outer Rim Territories, they receive what appears to be a distress signal from the planet Acheron also known as LV426. They are woken up from their hibernation in order to investigate the signal. While eating their first meal, "Mother" the Nostromo's main computer alerts Dallas that he has a message. While scanning the area, the crew begin to realize that they are not near Earth or in the Earth's solar system. Ripley tries to reach travel control, but cannot hail anyone. Lambert is able to pinpoint their location which is just shy of Zeta 2 Reticuli. Dallas tells the crew Mother has changed their course due to a transmission they received. Brett and Parker want to ignore the signal, however Ash tells them by law they are required to investigate any possible signal that could be of an intelligent origin.
They land on the planet LV426 to investigate the transmission, but suffer damage to the ship due to the rough landing. Kane volunteers to go search for the signal, and Dallas tells Lambert she will also be part of the search crew. The search crew find a large spaceship with a hole which they travel to. They climb into the cockpit area and find the petrified remains of a large alien pilot with a large hole in its chest, like something exploded from inside of him. Kane finds a hole in the deck of the ship which they search next. Ripley tells Ash, Mother has decided part of the transmission and it appears to be a warning rather than a distress signal. She wants to warn the search crew, however Ash doesn't see any point in it. Kane is lowered into the hole and finds a cargo bay with a large amount of leathery looking egg sacks or Spores. He prods at one of the eggs, it opens up revealing a life form inside. As Kane looks into the egg sack, a Facehugger jumps out and attaches to the face of Kane's spacesuit.
Dallas and Lambert return to the Nostromo carrying Kane. Dallas tells Ripley something has attached itself to Kane and he needs to be brought to the infirmary. Ripley refuses to open the hatch to let them in as she is worried Kane could have a potentially deadly life form and he should be quarantined, but Ash ultimately opens the hatch. Dallas and Ash take Kane to the infirmary and crack open his space helmet and see the full extant of the Facehugger for the first time. They do a scan of Kane and see that the Facehugger is feeding him oxygen to keep him alive. Dallas decides to risk removing the Facehugger and when they make an incision in one of its legs, acidic blood pours out and eats through a couple floors of the ship before drying up. Ripley confronts Ash about letting Kane into the ship, but he insists letting him in and putting the crew in possible danger was a risk he was willing to take. Later, Ash tells Dallas there has been a change in Kane's status and he and Ripley go to the infirmary. They find the Facehuffer gone from Kane's face, and when they search for it, they find it dead, but Kane alive. Brett and Parker repair the ship enough to take off and the crew returns to space. Lambert tells the crew they have another 10 months before they make it back to Earth. The crew is notified of a change in Kane's condition and when they go to the infirmary they find Kane sitting up and alert. They agree to have one more meal before they go into hibernation and during the meal Kane starts to scream and choke. The crew lay him on his back and a small Xenomorph explodes out of Kane's chest and runs away.
Kane's body is shot out into space and the crew decide to search for the Xenomorph. They use a mixture of a tracking device Ash develops and an electric prod Brett makes to try and find and chase the Xenomorph. They split into two groups, Dallas, Lambert and Ash and another group of Ripley, Brett and Parker. Ripley and her group detect a signal in the tracker and follow it into a crew cargo area. They open a door and Jones jumps out. Brett is forced to go after and catch Jones by the others so they wont get a signal from him versus the Xenomorph again. As Brett is searching for Jones he is attacked and grabbed by the Xenomorph. Ripley and Parker run to Brett's screams but by the time they get there he is pulled into the air ducts. Dallas decides on a plan of forcing the Xenomproph through the air ducts into the air lock and then shooting it into space. Parker makes a flame thrower and Dallas makes his way slowly into the air ducts. As he is making his way through the airlocks, Dallas is ambushed by the Xenomporph and grabbed. Ripley decides to continue with Dallas' plan and is upset that Ash can't provide any help in destroying the Xenomorph. Ripley asks Mother why the science division hasn't been able to help destroy the Xenomorph and she reads about Special Order 937 which states the priority of the Nostromo was to gather the alien life form and all crew were expendable. As she is trying to leave the area, Ripley is attacked by Ash. Parker and Lambert enter the room and Parker saves Ripley by knocking Ash's head from his body, revealing Ash to be a robot.
They plug Ash's head into a power system and Ripley asks Ash if there is any way they can kill the Xenomorph. Ash doesn't give them good odds of surviving and gives them his sympathies before smiling, and Ripley unplugs him. The remaining three members of the crew decide to take their chances in the escape shuttle after setting the self destruct system on the Nostromo. Ripley goes searching for Jones and when she finds him she puts him into a carrier. As Lambert and Parker are gathering supplies they are ambushed by the Xenomorph and both of them are killed. Ripley finds Lambert and Parker dead and sets the ship to self destruct. As Ripley is walking through the ship she finds Brett and Dallas cocooned and Dallas still alive. Dallas begs Ripley to kill him and she kills him with the flame thrower. As she is running back to the escape shuttle Ripley encounters the Xenomorph and she runs back to try and stop the self destruct sequence but runs out of time. Ripley goes back to the escape shuttle with Jones. Ripley uses the escape shuttle to fly away from the Nostromo as it self destructs. As Ripley is preparing to go into hibernation she sees the Xenomorph hiding in the ship. She quickly gets into a spacesuit and then goes to the ship controls. She opens the air shaft and the Xenomorph is eventually sucked out into space when Ripley shoots it with a dart from a dart gun. The Xenomorph tries to climb back into the escape ship, however Ripley turns on the engines and the Xenomorph is shot into space. Ripley records a final message relaying the details of the destruction of the Nostromo and its crew and she goes into the hibernation pod with Jones as the escape ship flies through space.