In the movie, a U.S. Marine spacecraft returning to Earth from a mission accidentally carries an alien egg, which hatches and causes an electrical fire. The alien attaches itself to one of the crew members, leading the ship’s system to eject an escape pod into space.

The pod lands on Fiorina 161, an all-male prison colony. The inmates discover the pod and rescue the sole survivor, Lieutenant Ellen Ripley, who is treated by the colony's chief medical officer, Dr. Clemens. Ripley learns she is the only survivor from the crash and soon discovers the dangerous situation that awaits.