A space mining outfit on their way home to Earth is woken out of a deep sleep by an SOS message. They are required to respond, so they change their course for a distant planet. When they arrive, they move down to the planet's surface to discover the source of the beacon. They discover some exotic, abandoned alien spaceship, and an enormous, ancient corpse of a creature with its chest bursted out. Deep within the spaceship, they discover an alien's nest. When one scientist examines an egg, it opens up, and the creature inside attacks him. It attaches itself to his face, and becomes unremovable. The other two crewmen manage to get him back to the ship but Ripley the ship's second officer in command doesn't want them to bring him aboard because of possible contamination. But Ash, the Medical Officer, opens the doors anyway, and lets the victim in. They put him in the infirmary, and learn that the hatchling is controlling his breathing patterns. After a while, the hatchling detaches itself.
Before going back into cryonic sleep, the crew decides to celebrate the recovery. But during the celebration, the victim goes into panic, and starts writhing in pain. His chest starts to warp, and then it bursts open, and a snake-like creature slithers out.
They begin searching for the creature, but before long, it has grown into a huge monster that resembles a 7 foot tall lobster with sharp teeth and claws, and it hunts down the crew, one by one. Ripley and Ash are two of the last survivors, and Ash attempts to kill Ripley, but Parker manages to bludgeon him, revealing that he's an android. When they get Ash's head working again, he reveals that his mission was to bring back a living alien for the Company to study.
Before long, Ripley, and a cat, are the only survivors. She decides to set the spaceship to self-destruct, taking the alien with it. The countdown begins, and she hurries to the escape pods, with the cat in hand. The pod ejects, back to earth.