In 2122, the commercial starship Nostromo is on a return trip to Earth with a seven-member crew in stasis. Receiving a transmission from a nearby moon, the ship's computer awakens the crew from stasis. The crew land on the moon to investigate the signal, damaging the ship in the process. While Captain Dallas, Executive Officer Kane and Navigator Lambert set out to find the source of the signal, Warrant Officer Ripley, Science Officer Ash, and engineers Brett and Parker remain within the ship.
The investigating team discover an alien spacecraft, as well as the corpse of an alien creature whose rib cage looks to have exploded from the inside. Back on the Nostromo, Ripley determines that the transmission may not have been a distress call, but a warning. While investigating the spacecraft, Kane discovers a chamber containing multiple eggs, one of which hatches an alien creature which attaches itself to Kane's face. Dallas and Lambert carry an unconscious Kane back to the Nostromo, where the crew tries unsuccessfully to remove the creature from Kane's face. Eventually, the creature detaches itself of its own accord and dies.
Kane later awakens, seemingly unharmed, but during a meal before re-entering stasis, a small alien creature bursts from Kane's chest, killing him. The creature flees into the ship and the crew attempt to capture it. During the search, Brett follows the crew's cat, Jones, into a large room, where he is attacked by the now fully-grown alien, who abducts him and vanishes. Dallas attempts to pursue the alien, but is killed. Lambert suggests that the crew escape in the Nostromo's shuttle, but Ripley explains that the shuttle will not support four people.
Accessing the ship's computers, Ripley learns that Ash was under secret orders to return the alien to the company, even at the expense of the crew. When she confronts Ash, he attempts to kill her, but Parker intervenes, decapitating Ash with a blow from a fire extinguisher, exposing Ash as an android. Ash taunts the remaining crew members about their impending deaths before being incinerated by Parker. The remaining crew members attempt to activate the Nostromo's self-destruct sequence and escape in the shuttle, but Parker and and Lambert are killed by the alien, leaving Ripley the sole survivor. Ripley activates the self-destruct sequence and narrowly escapes in the shuttle just as the Nostromo explodes, seemingly killing the alien.
As Ripley prepares for stasis, she learns that the alien has stowed away on the shuttle. Ripley blows the alien out through the shuttle's airlock, but it hangs onto the doorway. It attempts to crawl into one of the shuttle's engines, but Ripley ignites them, blasting the alien into space. After broadcasting a distress call, Ripley places herself and Jones into stasis for the return trip to Earth.