The crew of the commercial towing vessel Nostromo finds themselves being awoken from hyper-sleep prior to their arrival at Earth. The ship's computer has diverted them a mysterious planet, where it picked up an signal of unknown origin. After landing on the planet, the ship's captain, Dallas, it's navigator, Lambert, and the second-in-command Kane go to investigate, while the rest of the crew - Warrant Officer Ripley, Science Officer Ash, and the engineers Brett and Parker work on repairs and monitor Dallas's team's progress.
Dallas and company arrive at the source of the signal, and find a gigantic ship there. After investigating, they discover that the ship's crew is dead, and the hold is full of many strange eggs. When Kane goes to look closer, a creature (Facehugger) leaps from the egg, and wraps around his head, penetrating his helmet and implanting its ovipositor down his throat. Dallas and Lambert take Kane back to the ship, and against Ripley's objections, bring him aboard. The repairs are completed and the ship takes off. Once they were back in orbit and on their way, they try to remove the Facehugger from Kane and learn that it's clinging too tightly for them to pry it off, and it bleeds acid, so they can't cut it of without causing catastrophic damage to the ship. After they decide to put Kane into stasis, the facehugger releases Kane and dies. A little later Kane himself appears to recover. As they dine on their last meal before returning to hypersleep, the egg that the facehugger had implanted into Kane hatches, causing a young Alien to burst out of his chest (a Chestburster), killing Kane.
The crew proceeds to split into teams and search the ship for the Chestburster, using a motion detector designed by Ash. The team of Ripley, Brett, and Parker find the alien, only to make a horrible discovery, the Alien has grown dramatically larger, almost twice the size of a human. This discovery also comes at a terrible cost, as the alien kills Brett. The crew determines that the Alien is using the air ducts to move around, and the crew decides to use the air-ducts to force the Alien into the airlock and space it. Dallas volunteers to go into the air ducts with a motion detector and a flame thrower to track down the Alien. He too is killed by the Alien.
With Ripley in command now, she uses the ship's computer to try to figure out what to do about the Alien, and discover that the company they work for, Wayland-Yutani, was aware of the presence of the Alien on this planet, and had sent the ship there both to investigate, and to bring an Alien back. The ship's crew was considered expendable. As Ripley is reeling from this revelation, she makes another shocking discovery - Ash is an Android sent by the company to monitor the crew. Ash tries to kill her, but is stopped by Parker. The surviving crew decides to scuttle the ship and escape in the ship's escape pod. As Ripley sets up the self-destruct sequence and saves the ship's cat, Jonesy, Lambert and Parker are killed by the Alien. Ripley barely escapes before the ship is destroyed.
However, Ripley is not safe yet - the Alien is also on board the escape pod. Ripley dons a space suit and manages to knock the alien out into the vacuum of space. Upon achieving her final victory, Ripley enters the final log entry for the Nostromo and enters hyper-sleep with Jonesy.