In 2379, two hundred years after the events of Alien 3, military scientists on the space vessel USM Auriga create a clone of Ellen Ripley, designated Ripley 8, using DNA from blood samples taken before her death. The Xenomorph queen's DNA has been mixed in with Ripley's, so the clone grows up with an embryo inside it. The scientists extract the embryo, raise it, and collect its eggs while keeping Ripley 8 alive for further study. As a result of the Xenomorphs' DNA inside her, the clone has enhanced strength and reflexes, somewhat acidic blood, and a psychic link with the Xenomorphs. Additionally, the Xenomorph's genetic memory allows the clone to have some of Ripley's memories.
A group of mercenaries consisting of Elgyn, Johner, Christie, Vriess, Hillard and Call, arrives at Auriga on their ship Betty. They deliver several kidnapped humans in stasis. The military scientists use the humans as hosts for the aliens, raising several adult Xenomorphs for study.
The Betty crew soon encounters Ripley 8. Annalee Call recognizes her name and tries to kill her, suspecting that Ripley 8 may be used to create Xenomorphs, but the creatures have already been cloned. The Xenomorphs, having matured, escape confinement by killing off one of their own to use its acidic blood to burn through their enclosures, aware of their blood's acidity from said genetic memory. They then capture Dr. Jonathan Gediman and kill a second scientist. They damage the Auriga and kill some of those people who failed to evacuate, including General Perez and Elgyn. Another crew member is captured and cocooned for eggmorphing. Military scientist Dr. Wren reveals that the ship's default command in an emergency is to return to Earth. Realizing that this will unleash the Xenomorphs on Earth, Ripley 8, the mercenaries, Wren, a Marine named Distephano, and surviving Xenomorph host Purvis, decide to head for the Betty and use it to destroy the Auriga. Along the way, Ripley 8 discovers a laboratory containing the grotesque results of the previous seven failed attempts to clone Ellen Ripley. The surviving one begs Ripley 8 to euthanise her; she complies and then incinerates the lab and its contents.
