The commercial space tug Nostromo is on a return trip to Earth with a seven-member crew in stasis: Captain Dallas, Executive Officer Kane, Warrant Officer Ripley, Navigator Lambert, Science Officer Ash and two Engineers, Parker and Brett. Detecting a transmission from nearby moon LV-426, the ship's computer, Mother, awakens the crew. Company policy requires any potential distress signal be investigated, so they land on the moon, sustaining damage from its atmosphere and rocky landscape. Parker and Brett repair the ship while Dallas, Kane and Lambert head out to investigate. They discover the signal comes from a derelict alien ship and enter it, losing communication with the Nostromo. Ripley deciphers part of the transmission, determining it to be a warning, but cannot relay this information to those on the derelict ship.
Meanwhile, Kane discovers a chamber containing hundreds of large egg-like objects. When he touches one, a creature springs out, breaks through his helmet, and attaches itself to his face. Dallas and Lambert carry the unconscious Kane back to the Nostromo. As acting senior officer, Ripley refuses to let them aboard, citing quarantine regulations, but Ash overrides her decision and lets them inside. Ash attempts to remove the creature from Kane's face but stops when he discovers that its blood is an extremely corrosive acid. It later detaches on its own and is found dead. The ship is partly repaired, and the crew lifts off. Kane awakens with some memory loss but is otherwise unharmed. During a final crew meal before returning to stasis, he chokes and convulses. A small alien creature bursts from Kane's chest, killing him, and escapes into the ship.
The crew attempts to locate it with tracking devices and capture or kill it with nets, electric prods and flamethrowers. Brett follows the crew's cat Jones into a huge supply room, where the now fully-grown alien attacks him and disappears with his body. After a heated discussion, the crew decide the creature must be in the air ducts. Dallas enters the ducts, intending to force the alien into an airlock, but it ambushes and kills him. Lambert implores the others to abandon ship and escape in its small shuttle but Ripley, now in command, explains it will not support four people and says they will continue the plan of flushing out the alien.
Now with access to Mother, Ripley discovers Ash has been secretly ordered by the company to bring the alien back, with the crew deemed expendable. She confronts Ash, who tries to choke her to death. Parker intervenes and clubs Ash, knocking his head loose and revealing him to be an android. Ash's head is reactivated, and they learn he was assigned to ensure the creature's survival. He expresses admiration for the creature's psychology, unhindered by conscience or morality, and taunts them about their chances of survival. Ripley cuts off his power; as they leave, Parker incinerates him.
The remaining crew decides to self-destruct the Nostromo and escape in the shuttle. Parker and Lambert are killed by the creature as they gather supplies. Ripley initiates the self-destruct sequence, but finds the alien blocking her path to the shuttle. She retreats and attempts unsuccessfully to abort the self-destruct. With no further options, she makes her way to the shuttle and barely escapes as the Nostromo explodes.
As Ripley prepares for stasis, she discovers that the alien is aboard, having wedged itself into a narrow space. She puts on a spacesuit and uses gas to flush the creature out. It approaches Ripley, but before it can attack she opens an airlock door, almost blowing the creature into space. It manages to hang on by gripping the frame. Ripley shoots it with a grappling hook, but the gun catches as the airlock door closes, tethering the alien to the shuttle. As it floats into one of the engine exhausts, Ripley ignites them to blast the creature free. After recording the final log entry, she places herself and Jones the cat into stasis for the trip home to Earth.