In deep space, the crew of the commercial starship Nostromo is awakened from their cryo-sleep capsules halfway through their journey home to investigate a distress call from an alien vessel. After landing on a barren planet, Warrant Officer Ripley works to decipher the transmission and discovers that the signal is actually a warning, not an SOS. But it is too late to turn back as three members of the crew have already left to investigate the derelict ship. The terror begins when Kane encounters a nest of eggs inside the alien ship. An organism from inside an egg leaps out and attaches itself to his face, causing him to fall into a coma. Because of the facehugger's awesome defenses, the crew of the Nostromo cannot remove it without also killing Kane. As the crew searches for a way to save Kane, the facehugger dies and Kane wakes up, feeling fine. He is brought out of isolation and the crew soon makes another horrifying discovery - the lifeform they've found breeds within a human host.

The young alien that began its life inside of Kane escapes the astonished crew and hides in the Nostromo where it continues to develop. As the crew attempts to hunt it down, it in turn hunts them. While searching for a safe escape, Ripley discovers the Company's secret plans to bring the alien to Earth and that Ash, the Nostromo's Science Officer, is willing to kill his fellow crewmates to achieve this goal. Warrant Officer Ripley ultimately finds that the alien must be destroyed, not only to save herself and her crew, but all of mankind. She sets the Nostromo to self-destruct and prepares to escape in a shuttle, leading to the final explosive battle for survival.