The original script's working title was Star Beast. Writer Dan O'Bannon wasn't too happy with it and adopted the iconic title after noticing how many times the word "Alien" appeared in the text.

The commercial space tug Nostromo is on a return trip back to Earth with a seven-member crew in stasis.

Owing to Jonesy's small size and mass, he can comfortably share a hypersleep capsule with one of the crew members during the long journey.

Detecting a transmission from nearby moon LV-426, the ship's computer, Mother, awakens the crew.

Executive Officer 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 explains to Ripley: "I went out five times with another science officer. They replaced him two days before we left Thedus with Ash."

Citing quarantine regulations, Ripley refuses to let Dallas and Lambert aboard with the injured Kane, but Ash overrides her and lets them inside.

The creature detaches from Kane's face, and he seems to recover, but during a final crew meal before returning to stasis, he begins to choke and convulse. A small alien creature bursts from Kane's chest, killing him, and escapes into the ship.

Captain Arthur Dallas enters the ducts with a flamethrower, intending to force the alien into an airlock, but it ambushes and kills him.

When Ripley discovers that Ash has been secretly ordered by the company to retrieve the Alien, and that the crew has been deemed expendable, she confronts him. Ash tries to choke Ripley, but Parker clubs him, knocking his head loose and revealing him to be an android.