Spoiler
When she left Earth, Ellen Ripley promised her daughter Amanda she would return home for her 11th birthday. Amanda never saw her again.
Fifteen years later (2137), Weyland-Yutani Technician Amanda Ripley is approached by the synthetic Christopher Samuels, a Weyland-Yutani executive, who informs her that the flight recorder unit of the Nostromo was recently located and is being held aboard Sevastopol Station, a remote freeport space station owned by the Seegson Corporation in orbit around the gas giant KG348. Samuels offers to allow Amanda to be a part of the Weyland-Yutani team sent to retrieve it, so that she can have closure regarding the fate of her missing mother. Ripley, Samuels, and Weyland-Yutani lawyer Nina Taylor travel to Sevastopol onboard the cargo ship Torrens, owned by Captain Diane Verlaine. The group arrive at Sevastopol to find the station damaged and communications dead. Ripley, Samuels, and Taylor attempt to spacewalk over to the station to investigate, but their EVA line is severed by debris from an explosion and Ripley is separated from Samuels and Taylor.
Ripley manages to make her way inside the station and finds a complete breakdown of civil society, with the station's inhabitants reduced to small groups of frightened, paranoid looters hoarding scavenged resources and shooting non-group members on sight. She is briefly held at gunpoint by Axel, but convinces him to help her in exchange for a ride off the station aboard the Torrens. Axel explains the current situation is due to a "monster" loose aboard the station, and, after the 2 escape from a group of hostile survivors, an Alien appears and kills Axel. Ripley evades the Alien and contacts Samuels, and learns that Taylor was injured, forcing Ripley to retrieve medical supplies from the San Cristobal Medical Facility to treat her.
Upon returning to Samuels and Taylor with the medical supplies, the group is joined by Colonial Marshal Waits, and his deputy, Ricardo. Waits explains that the Alien was brought on board the station by Marlow, the captain of the salvage vessel Anesidora, the ship that found the Nostromo flight recorder. Ripley speaks with Marlow, who is kept in a cell by Waits, and learns that the Anesidora crew discovered the derelict on LV-426 by following the Nostromo's flight route, and Marlow's wife, Foster, was impregnated by a Facehugger in the ship's cargo hold. Marlow brought her aboard Sevastopol for treatment, and the Alien hatched from her and began to wreak havoc onboard. Waits convinces Ripley to help him contain the Alien by luring it to the Gemini Exoplanet Solutions module, a remote section of the station, then sealing it inside. However, once the Alien is contained, Waits ejects the module from the station with Ripley still inside. She manages to find an EVA suit and eject herself from the module and back to Sevastopol, while the Alien is left behind in the jettisoned module as it falls into KG348.
With the Alien disposed of, the situation aboard the station appears back under control, but the station's android workforce abruptly begins hunting down and killing the station's human inhabitants, including Waits and his remaining men, though Ricardo survives. Samuels attempts to interface with the station's controlling artificial intelligence, APOLLO, and get it to stop the slaughter. Despite Ripley's attempts to save him, APOLLO's defensive countermeasures end up killing Samuels, but not before he opens a path for Ripley to APOLLO's control chamber.
After evading more androids and reaching the control chamber, Ripley discovers that Sevastopol was recently purchased by Weyland-Yutani, who, by Special Order 939, instructed APOLLO to protect the Alien at all costs so the company could acquire it, even if it meant killing the station's crew. Ripley demands to know why APOLLO is continuing to carry out this directive even though the Alien is no longer aboard the station. APOLLO directs her to the station's reactor core, where Ripley discovers a "nest" with multiple Aliens, Facehuggers and Eggs. Ripley initiates a reactor purge to destroy the nest, but multiple Aliens manage to escape into the station.
Ripley learns from Ricardo that Taylor was sent by Weyland-Yutani to retrieve the Alien, and that she freed Marlow in exchange for the location of LV-426. However, Marlow took her hostage and fled to his ship. Ripley uses an ambulance shuttle to pursue them in hopes of using the Anesidora to escape the station. However, Marlow, who has the Nostromo flight recorder data, shows Ripley a message for her from her mother regarding her motives for destroying the ship Marlow reveals he is attempting to detonate the Anesidora's fusion reactor to destroy the station and his ship, ensuring the Aliens do not come into contact with the rest of humanity. Taylor hits Marlow with a wrench while he is arguing with Ripley, and both work together to try to prevent the reactor explosion. They are only partially successful, however, with Taylor being killed by an electrical overload and Ripley barely escaping as the ship explodes. While the explosion does not destroy Sevastopol, it damages the station's gravity stabilizers, causing it to begin falling into KG348's atmosphere.
Ripley and Ricardo realign Sevastopol's antenna to contact the Torrens for evacuation, but Ricardo is attacked and subdued by a Facehugger. Ripley attempts to put on an EVA suit, preparing to release the clamps securing the Torrens to Sevastopol, but gets captured and cocooned by an Alien. She escapes, dons an EVA suit and releases the clamps just as Sevastopol is destroyed. Aboard the ship, Ripley abruptly loses contact with Verlaine, and subsequently discovers an Alien has boarded the ship as well, cornering her in the airlock. Still in her EVA suit, Ripley hits the a button to open the airlock, jettisoning both herself and the Alien into space. The game ends with Ripley adrift in space, waking up to the light of a vessel approaching her.
