In the year 2122, the crew of a space craft called the Nostromo are awakened from their stasis sleep after the ship detects what might have been a distress signal from a small planetoid. The Nostromo is crewed by 7 astronauts on a corporately owned expedition, which is returning to Earth when the movie begins. Picking up the unexpected transmission, the crew members decide to explore the source of the signal. Warrant Officer Ripley (Sigourney Weaver) quickly realizes that the signal was not a distress signal at all, but a warning from an alien space craft, as the Nostromo lands on the stormy planet.
On the alien craft, Executive Officer Kane discovers a chamber filled with unhatched eggs. While examining one egg, Kane is attacked by a small alien that attaches to his face. Dallas and Kane return the unconscious Kane to the Nostromo. Ripley invokes protocol, wanting to prevent Kane from coming back onto the ship for 24 hours, but Science Officer Ash overrides her quarantine and allows the three to re-board the ship. Kane is placed in the infirmary, where they find an alien creature attached to his face.
Kane awakens and the alien appears to have died. Kane is believed to be recovered from the incident and the crew sits down to a meal. However, during the meal, he experiences severe stomach pains, and then, in one of the most iconic scenes in contemporary film, an alien erupts from his stomach, escaping into the Nostromo. The crew debates how to exterminate the creature, worrying about the safety of the ship. They decide to try to capture the alien and burn it.
Brett, an engineer on the Nostromo, is the first crew member attacked by the fully-grown alien, which grows rapidly into a giant, blood-thirsty beast. Upon Brett's death, the crew agrees to eject the monster from the ship, rather than bring the alien back to Earth for research or possible corporate gain. A race to contain the alien ensues, but the alien continues to kill crew members ruthlessly. After Brett and Captain Dallas are both killed by the alien, Ripley tries to find a way to dispose of the alien once and for all. While troubleshooting on the computer system of the ship, called MUTHER, she learns that Ash has been employed to bring the alien back to Earth for research and potential profit, no matter the cost. MUTHER informs her that the system is programmed to return the alien back to Earth, even if it is at the expense of the lives of the crew members. When Ripley interrogates him, Ash attacks her. Engineer Parker and crew member Lambert rush to Ripley's aid, decapitating Ash and revealing that he is an android put aboard the ship to ensure the retrieval of the alien and its safe transport back to Earth.
The three remaining crew members prepare to self-destruct the ship and escape on a small shuttle. Lambert and Parker are killed while gathering supplies for the escape, and Ripley and Jones, a cat, are the final remaining crew members. They escape by way of the shuttle, leaving the Nostromo to self-destruct, but Ripley is horrified to find that the alien snuck onto the escape shuttle. After a struggle, Ripley uses the engines to send the creature flying into space before setting course for Earth.