The Nostromo, a commercial towing spaceship, embarks on its journey back to Earth after hauling a massive refinery along with twenty million tons of mineral ore. The ship's crew, consisting of seven members, is in stasis during the return trip. Suddenly, an unknown transmission from a nearby Planetoid prompts the ship's computer to wake the crew. Following orders from their corporate employers, the crew detaches the Nostromo from the refinery to investigate, leading to some damage upon landing.
Captain Dallas (Tom Skerritt), Executive Officer Kane (John Hurt), and Navigator Lambert (Veronica Cartwright) venture out to trace the signal's origin. Meanwhile, Warrant Officer Ripley (Sigourney Weaver), Science Officer Ash (Ian Holm), and Engineers Brett (Harry Dean Stanton) and Parker (Yaphet Kotto) remain on the ship to oversee repairs and monitor the situation.
Upon their investigation, the trio stumbles upon a decrepit alien spacecraft, inside which they discover the remains of a colossal alien creature, its ribs grotesquely exploded outward. As the Nostromo's computer decodes the signal, Ripley determines it to be a warning. The crew then finds a vast chamber filled with alien eggs, one of which attaches itself to Kane’s face. They bring the unconscious Kane back aboard, where Ash allows them in, defying Ripley’s quarantine protocols. Initial efforts to remove the creature fail as it releases corrosive acid, but eventually the creature drops off and is found lifeless.
After repairs are completed, the crew prepares to resume their journey. However, when Kane regains consciousness, he unexpectedly chokes during a meal, and an alien creature bursts forth from his chest, leading to his death and the creature’s flight through the ship. Armed with makeshift weapons like motion trackers, electric prods, and flamethrowers, the crew tries to hunt down the Alien. The ship’s cat unexpectedly draws Brett into danger, resulting in him being killed and disappeared into the ship’s air shafts.
Dallas enters the air shafts in a desperate attempt to flush the creature out into space, but he too falls victim to the Alien. Lambert urges the remaining crew to evacuate in the shuttle, but Ripley, now taking charge, explains the shuttle cannot accommodate all four of them.
Through a harrowing realization, Ripley discovers that Ash is a robot created to ensnare the Alien and return it to corporate masters, even at the cost of the crew’s lives. When Ash attacks Ripley, Parker intervenes, taking the android down. As they plan to arm the Nostromo’s self-destruct mechanism to escape, Parker and Lambert meet their gruesome end at the hands of the Alien while gathering supplies.
In a frantic effort, Ripley kicks off the self-destruct sequence and rushes to the shuttle, only to find the Alien blocking her escape. Unable to abort the self-destruct, she narrowly escapes as the Nostromo detonates behind her.
However, her trials are far from over. As she prepares to enter stasis, Ripley discovers the Alien has snuck aboard the shuttle. In a desperate move, she dons a space suit and opens the hatch, resulting in explosive decompression that thrusts the creature out. Although she temporarily succumbs to chaos, she utilizes a grappling gun to propel the Alien further from the shuttle. Unfortunately, the gun gets caught during the hatch’s closure, tethering the creature as it desperately attempts to re-enter the ship. With quick thinking, Ripley fires up the engines, blasting the Alien into the vast unknown of space. Finally, she secures herself and the ship's cat into stasis, beginning their long journey back to Earth.
