The warship USS Sulaco, back from its mission on Acheron, drift into space in direction of Earth. The survivors of the mission are Ellen Ripley, the Corporal Hicks, the little girl Newt, the andoïd Bishop and... an alien egg that belongs with them. This one opens and attacks one of the survivors in suspended animation. This causes a procedure of urgent evacuation of the ship.
The rescue capsule crushes on the planet Fury 161, a penitentiary planet where it is recuperated by fearsome captive criminals. Ripley is the only to survive of the capsule's crash. She is transferred in the prison, leaving behind her the Alien in incubation...
Ripley whose cranium has been shaved for sanitary reasons, returns later to the capsule and understands that an Alien has followed her. She orders the incineration of the dead bodies. But it is already too late, the Alien lays in the body of an errant dog and invests the center.
Most of the prisoniers are killed by the Alien. To the continuation of the deterioration of her health, Ripley discovers that she wears a germ of the beast in her body. It concerns a queen. She understands now why the Alien that ravages the penitentiary did not attack her.
Waiting for external help that do not arrive, she decides to kill the monster. The hunting is all the more difficult that the prison don't possess any armament. The only solution is to attract the beast to the metal foundry and to drown it under the lead.
The rescue composed with scientists as well as a surgical antenna arrive on the planet Fury 161...
Click to read the end of the story. Spoiler ahead !
The company, aware of Ripley's secret, wishes to return on Earth the embryo. She finaly get rid of the monster and prefers to commit suicide (anyway a quasi certain death) in the foundry, entailing with her the monster that bursts from her chest, rather than to give to the company the unconsciousness to make proliferate the Alien's race on Earth.