Onbard the medical reseach army spaceship "USM Auriga", led by general Perez, scientists have succeeded, after many unfruitful tests, unauthorized cloning of the lieutenant Ellen Ripley, pregnant of a alien queen, from her deep-frozened blood. The beast, extracted during of a surgical intervention, has now arrived to maturity and has begun to lay his eggs.
Against all wait, Ripley's clone has preserved the memory of the officer died on the penitentiary planet Fiorina 161. Created from a genetic crossing with the Alien, she possesses a highly developed senses, forces and instincts of the terrible creature. Her acidic blood is elsewhere one of the best evidences of her inheritance. Considered as a lost package, she lives now imprisoned as a laboratory animal.
Perverted by money making, Perez, in total illegality, called a group of mercenaries to steal minors's cryo-tubes, still in state of stasis, subtracted by act of piracy to a commercial freighter. Onboard the buccaneer spaceship "Betty", Elgyn, Christie, Johner, Call, Vriess and Hillard come to deliver the precious merchandise necessary for the alien final insemination phase. But Call's personal intention, a beautiful and fragile young woman recently enrolled by the plunderers, is other. When she attempts to kill the mother's monster, not only she doesn't succeed, but her illegal intrusion in the woman's cell provokes the soldiers's aggressiveness.
The situation poisons and the mercenaries, confused as terrorists, kill the soldiers who hold their weapons on them. Meanwhile, the monsters that are grown, flowered from unconscious minor's thoraxes, escape the security cells, not hesitating to kill each another to escape. On the Auriga, death begins to spread.
Pirates decide to return to the Betty and take with them physician Wren and the soldier Distephano in hostage to cover their retirement, without taking the real measures of the announced catastrophe. A dozen of Aliens knock on all levels of the spaceship. The emergency procedure urgently turns to the massacre. After Elgyn's death, they all have to work together to survive. Despite Call's reticence, Ripley joins the group and learns that the Auriga, consequently of the emergency procedure, has naturally set course to the Earth that she will reach in three hours, an unacceptable script.
During their escape, Ripley discovers with horror the cloning laboratory and the monstrous and missed attempts of her own rebirth. All equally testify the wideness of the minors' massacre whose cryo-tubes have been given in pasture to the Aliens. Although infected, one of them, Purvis, is still alive. Later runaways fall in a trap where alien eggs are waiting for them...
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On the way to escape the terrible ambush, fugitives are betrayed by Wren, who precipitates Christie and Call's deaths before to escape. At the general surprise, Call returns to life because... she is a droïd. Ripley profits of the appropriateness to connect Call to the central module of the Auriga. Unable to release release the ship self-destruction, she programs her to crush on the Earth and starts the Betty takeoff procedure.
If the last survivors succeed in joining the saving spaceship, it is not the same story for Ripley. The woman falls in the nest of the queen. Endowed with a new reproductive system, Ripley's genetic present, the former comes now to give birth to a sanguinary monster half-man half-alien that, seeing in Ripley its real mother, destroys the queen. Ripley, profiting a moment of inattention of the beast, joins the Betty and her last survivors; Call, Vriessn and Johner. When the monster climbs clandestinely onboard, Ripley expels it in space, not without regret, instinctive maternal obliges!