The commercial towing spaceship Nostromo is en route back to Earth carrying twenty million tons of mineral ore when its central computer, "Mother," awakens the seven-member crew from stasis prematurely. Mother has intercepted an unknown acoustic transmission from a nearby uncharted moonoid, LV-426. Under strict company contract regulations, any signal resembling a potential distress beacon must be investigated under penalty of total forfeiture of pay.
Captain Dallas, Executive Officer Kane, and Navigator Lambert don spacesuits and trek across the stormy, desolate surface to find the source: a colossal, bio-mechanical derelict alien spacecraft. Inside, Dallas and Lambert discover the fossilized skeleton of a giant humanoid pilot whose ribcage appears to have exploded outward from within. Meanwhile, Kane descends into a lower cargo hold filled with thousands of leathery, egg-like pods. When Kane inspects one, it opens, and a multi-legged organism violently bursts out, melting through his helmet visor and latching onto his face.
Dallas and Lambert carry the unconscious Kane back to the ship. Warrant Officer Ripley refuses to let them board, citing mandatory 24-hour quarantine protocols to protect the crew, but Science Officer Ash overrides the airlock controls and lets them in. In the medical bay, Ash and Dallas discover that the organism is pumping oxygen down Kane's throat while its acidic blood melts through two ship decks when they attempt to surgically remove it. Hours later, the facehugger detaches on its own and dies. Kane regains consciousness, apparently healthy, with zero memory of the encounter.
During a final meal before returning to cryo-sleep, Kane suddenly convulses in agony. As the horrified crew tries to restrain him, his chest cavity rips open from the inside, and a small, serpentine alien creature emerges through his ribcage, hissing before scampering into the dark corridors. Brett, Parker, and Ripley fashion cattle prods, nets, and primitive motion trackers to hunt the beast. Brett tracks the ship's cat, Jonesy, into a cooling room, where he encounters the creature, now molted into a massive, eight-foot-tall bipedal monster with a phallic head and extending jaw. The alien kills Brett and drags his body into the air ducts.
Dallas enters the duct system with a flamethrower to flush the creature into an airlock, guided by the motion tracker. The tracker glitches, revealing the alien is closing in rapidly; Dallas turns into the shadows and is instantly ambushed. With Dallas gone, Ripley takes command and accesses Mother to ask for advice. She discovers "Special Order 937": the Weyland-Yutani Corporation knew about the organism all along and sent the crew to retrieve it for biological weapons research, stating that the crew is completely expendable. Ash confronts Ripley and attacks her, revealing he is an undercover android planted by the company to safeguard the specimen. Parker and Lambert intervene, decapitating Ash with an oxygen cylinder and fire extinguisher.
Ripley reactivates Ash's head for interrogation; the android coldly praises the creature's purity, describing it as a perfect organism unburdened by conscience or remorse, before Parker destroys him permanently. Ripley, Parker, and Lambert decide to scuttle the Nostromo using its self-destruct sequence and flee in the small escape shuttle, Narcissus. While gathering coolant canisters, Lambert freezes in terror as the alien corners her, and Parker is killed trying to save her before Lambert is slaughtered off-screen.
Ripley races through the flashing corridors, activates the emergency self-destruct mechanism, and grabs Jonesy the cat. Finding the alien blocking the path to the shuttle, she tries to abort the detonation, but Mother confirms the countdown is irreversible. Ripley makes a desperate dash, narrowly makes it into the shuttle, and launches just as the Nostromo explodes in a colossal fireball.
