The commercial towing spaceship Nostromo is on a return trip from Thedus to Earth, hauling a refinery and twenty million tons of mineral ore, and carrying its seven-member crew in stasis. Upon receiving a transmission of unknown origin from a nearby planetoid, the ship's computer awakens the crew. Acting on orders from their corporate employers, the crew detaches the Nostromo from the refinery and lands on the planetoid, resulting in some damage to the ship. Captain Dallas (Tom Skerritt), Executive Officer Kane (John Hurt), and Navigator Lambert (Veronica Cartwright) set out to investigate the signal's source while Warrant Officer Ripley (Sigourney Weaver), Science Officer Ash (Ian Holm), and Engineers Brett (Harry Dean Stanton) and Parker (Yaphet Kotto) stay behind to monitor their progress and make repairs.
Dallas, Kane, and Lambert discover that the signal is coming from a derelict alien spacecraft. Inside it they find the remains of a large alien creature whose ribs appear to have been exploded outward from the inside. Meanwhile, the Nostromo's computer partially deciphers the signal transmission, which Ripley determines to be some type of warning. Kane discovers a vast chamber containing numerous eggs, one of which releases a creature that attaches itself to his face. Dallas and Lambert carry the unconscious Kane back to the Nostromo, where Ash allows them inside against Ripley's orders to follow the ship's quarantine protocol. They unsuccessfully attempt to remove the creature from Kane's face, discovering that its blood is an extremely corrosive acid. Eventually the creature detaches on its own and is found dead. With the ship repaired, the crew resume their trip back to Earth.
Kane awakens seemingly unharmed, but during a meal before re-entering stasis he begins to choke and convulse until an alien creature bursts from his chest, killing him and escaping into the ship. Lacking conventional weapons, the crew attempt to locate and capture the creature by fashioning motion trackers, electric prods, and flamethrowers. Brett follows the crew's cat into a large room where the now-fully-grown Alien attacks him and disappears with his body into the ship's air shafts. Dallas enters the shafts intending to force the Alien into an airlock where it can be expelled into space, but it ambushes him. Lambert implores the remaining crew members to escape in the ship's shuttle, but Ripley, now in command, explains that the shuttle will not support four people.
Accessing the ship's computer, Ripley discovers that Ash has been ordered to return the Alien to the Nostromos corporate employers even at the expense of the crew. Ash attacks her, but Parker intervenes and decapitates him with a blow from a fire extinguisher, revealing Ash to be an android. Before Parker incinerates him, Ash predicts that the other crew members will not survive. The remaining three crew members plan to arm the Nostromos self-destruct mechanism and escape in the shuttle, but Parker and Lambert are killed by the Alien while gathering the necessary supplies. Ripley initiates the self-destruct sequence and heads for the shuttle with the cat, but finds the Alien blocking her way. She unsuccessfully attempts to abort the self-destruct, then returns to find the Alien gone and narrowly escapes in the shuttle as the Nostromo explodes.
As she prepares to enter stasis, Ripley discovers that the Alien is aboard the shuttle. She puts on a space suit and opens the hatch, causing explosive decompression which forces the Alien to the open doorway. She shoots it with a grappling gun which propels it out, but the gun is yanked from her hands and catches in the closing door, tethering the Alien to the shuttle. It attempts to crawl into one of the engines, but Ripley activates them and blasts the Alien into space. She then puts herself and the cat into stasis for the return trip to Earth.