The movie starts right at the ending of the second movie where Ellen Ripley and three survivors start to travel back home. Unfortunately, they crash-land on a prison planet and Ripley discovers that her companions are dead because an embryonic alien infiltrates the starship. The alien accidentally triggers the ship's emergency systems, dropping the escape capsule to the surface of the planet. Ripley finds herself in a prison colony peopled by a religious cult composed of former murderers and rapists. Meanwhile, the alien has managed to grow into a new and deadly form, and is picking off the prisoners lacking weapons. Much to her horror, Ripley soon discovers that the real danger is much more personal.