The commercial space tug Nostromo is returning to Earth with a seven-member crew in stasis: Captain Dallas, Executive Officer Kane, Warrant Officer Ripley, Navigator Lambert, Science Officer Ash, and engineers Parker and Brett. The ship's computer, Mother, detects a transmission from a nearby moon and awakens the crew. Following company policy to investigate "any systematised transmission indicating intelligent origin", they land on the moon and Dallas, Kane, and Lambert head out to investigate the signal's origin. The party discovers that it is from a derelict alien spaceship; inside, they find the remains of a large alien with a hole in its torso. Mother later deciphers part of the transmission, which Ripley determines is actually a warning message.
Kane discovers a chamber containing hundreds of large eggs. When he touches one, a creature springs out, penetrates his helmet, and attaches itself to his face. Dallas and Lambert carry the unconscious Kane back to the Nostromo. As the acting senior officer, Ripley refuses to let them aboard, citing quarantine regulations, but Ash overrides her decision and lets them inside. The crew returns to space, and Ash attempts to remove the creature from Kane's face, but stops when he discovers that its extremely corrosive acidic blood could hurt Kane and potentially damage the ship's hull. It eventually detaches itself and is found dead. Kane awakens with some memory loss but otherwise seems fine. During a final crew meal before returning to stasis, he suddenly chokes and convulses. A small alien creature bursts from Kane's chest, killing him, and escapes into the ship.
After ejecting Kane's body out of an airlock, the crew attempts to locate the creature with tracking devices and kill it. Brett follows the crew's pet cat, Jones, into a landing leg compartment,[10] where the now fully-grown alien attacks Brett and disappears with his body. The crew determines the creature must be in the air ducts. Dallas enters the ducts with a flamethrower, intending to force the monster into an airlock, but it ambushes and seemingly kills him. Lambert implores that they abandon ship and flee in an escape shuttle, but Ripley, now in command, explains it will not support four people and insists on continuing Dallas' plan of flushing out the alien.
Accessing Mother, Ripley discovers the company has secretly ordered Ash to return with the alien for study, and to consider the crew expendable. She confronts Ash, who tries to kill her, but Parker intervenes and clubs Ash, knocking his head loose and revealing him to be an android. The survivors reactivate Ash's head, who confirms the company's orders and states that the creature is unkillable, while expressing his admiration for it and taunting them about their chances for survival. Ripley shuts him down and Parker incinerates him.
The remaining crew decides to self-destruct the Nostromo and escape in the shuttle. However, Parker and Lambert are ambushed and killed by the creature while gathering supplies. Ripley initiates the self-destruct sequence but finds the alien blocking her path to the shuttle. She retreats, attempts unsuccessfully to abort the self-destruct, flees back to the shuttle with Jones, and narrowly escapes as the Nostromo explodes.
As Ripley prepares for stasis, she discovers that the alien is aboard, having stowed itself into a narrow compartment. She dons a spacesuit and flushes the creature out. It approaches Ripley, but before it can kill her, she opens an airlock door. The alien manages to hang on, but Ripley fires a grappling hook gun to push it out and then fires the engines, blasting it away into deep space. After recording her final log entry, she places Jones and herself into stasis for the trip back to Earth.
