The crew of the commercial spaceship Nostromo are on their way home when they pick up a distress beacon from a distant moon. The crew are under obligation to investigate and the spaceship descends on the moon afterwards. After a rough landing, three crew members leave the spaceship to explore the area on the moon. At the same time as they discover a hive colony of some unknown creature, the ship's computer deciphers the message to be a warning, not a distress call. When one of the eggs is disturbed, the crew realizes that they are not alone on the spaceship and they must deal with the consequences.