Sometime in the future a space ship and its crew land on a mysterious planet. While exporing the planet one the crew members is attacked by what seems to be some kind to plant that attaches to his face. Back on board he undergoes what appears to be successful surgery to detach the strange plant. However, later he begins to have what appears to be a seizure. In a now famous scene a small animal, the alien of the title, bursts from his stomach and then runs off to hide in the vents of space ship. The alien, now loose on the ship, grows at an alarming rate and begins picking off the remaining members of the crew one by one. Finally, only, Ridley (Sigourney Weaver) remains to fight the alien.