The commercial space vehicle Nostromo is headed back to Earth, but its onboard computer, "Mother," has received a mysterious beacon and thus decided to awaken the crew from their deep, interstellar sleep. Following their directive, the ship's captain, Dallas (TOM SKERRITT), decides they must investigate and thus they set down on an inhospitable planet.
There, Dallas and two crewmembers, Kane (JOHN HURT) and Lambert (VERONICA CARTWRIGHT), go to investigate and discover an ancient spaceship and the fossilized remains of a huge creature. They also discover an array of egg pods from which springs a small alien that attaches itself to Kane's faceplate.
He's rushed back to the ship, but Ripley (SIGOURNEY WEAVER) won't allow him in, knowing it will be breach the ship's quarantine. Science officer Ash (IAN HOLM), however, disregards that and Kane is saved. When the alien then appears to have died, the crew, including mechanics Brett (HARRY DEAN STANTON) and Parker (YAPHET KOTTO), prepare for another ten months of deep sleep.
A new alien, however, changes their plans when it literally pops out on the scene and flees somewhere into the massive ship. From that point on, the crew tries to avoid the alien that has acid for blood, is progressively growing larger by the minute and seems determined to kill all of them.