In the year 2122, the crew of the 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 ship is crewed by seven and is a corporately owned expedition which is returning to Earth when the movie begins. A small crew decides to explore the source of the signal. But, Warrant Officer Ripley, (Sigourney Weaver) quickly realizes that the signal was not a distress signal. It was a warning from an alien space craft.
On the alien craft, a room of eggs is discovered by Executive Officer Kane, who is attacked by a small alien that attaches to his face. He is returned unconscious to the Nostromo by Dallas and Lambert. Ripley cites protocol which should prevent Kane's return to the ship, but Science Officer Ash overrides her quarantine and allows the three to re-board the ship.
Kane awakes and is believed to be recovered from the incident. But, during a meal, he starts feeling 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 capture the alien and burn it.
Brett is the first crew member attacked by the fully-grown Alien. Upon his death, the crew decides to eject the monster from the ship, thus sacrificing chances to study the alien or perhaps use it for corporate gain. A race to contain the Alien ensues, as the crew seeks to control it, but the Alien continues to kill crew members. It is later discovered that Ash has been employed by the executives on board to subvert these plans so that the corporation can profit by owning the alien. When Ripley interrogates him, Ash attacks her and is decapitated, revealing that he is an android, who was put on board to ensure the capture of the Alien.
Lambert and Parker are killed while gathering supplies for their escape, and Ripley and Jones are the final crew members alive. They escape by way of the shuttle, leaving the Nostromo to self-destruct, but the Alien threatens to invade the shuttle. Finally, Ripley uses the engines to send the creature into space before setting course for Earth and placing her and Jones back into stasis.