||November 17, 2002
Bounty hunter Samus Aran, the protagonist, explores the surface of the planet SR388 with a survey crew from Biologic Space Laboratories (BSL). She encounters an X Parasite, virions that can replicate their hosts' physical appearances and memories, killing them afterward. Samus is unknowingly infected by the virus, and proceeds to return to the BSL station, when she suddenly loses consciousness and... See more is ejected from her ship before it crashes into a nearby asteroid belt. The Galactic Federation recovers her body and discovers that the X Parasite infected Samus's central nervous system. They counteract the parasite's damage by creating a vaccine using Samus's cellular sample from the remains of a Metroid that Samus previously contacted. The vaccine cures Samus and also gives her Metroid-like characteristics, including the ability to absorb and use the nuclei of X Parasites for nourishment, and the Metroid's vulnerability to extremely cold temperatures. Her infected Power Suit is sent to the BSL station for examination, although parts of the suit are bonded to her body and cannot be removed, forming a partially-armored suit known as the eponymous Fusion Suit. The Federation sends Samus to investigate an explosion aboard the BSL station. While there, she learns that the space station is swarming with organisms that are infected with X Parasites. A group of X Parasites manage to recover and use components of Samus's infected Power Suit to transform into the SA-X, a form of the parasite that partially mimics Samus at full power and possesses the Ice Beam that can exploit Samus's weakness to cold temperatures. Samus escapes from the SA-X before it attacks her, and continues exploring the space station. She discovers a restricted lab, which the Galactic Federation is secretly using to breed Metroids. Suddenly, the SA-X appears once again, and attempts to destroy the lab, with itself in it. Samus narrowly escapes the lab before it is destroyed, and later learns that the SA-X managed to asexually reproduce before perishing, subsequently cloning itself. In addition, Samus learns that the Galactic Federation plans to capture the X and the SA-X for further study. Samus is certain that the Federation underestimates the destructive power of the X Parasites, and takes it upon herself to destroy the BSL station by setting it on a collision course with SR388. As she prepares to exit the station, she encounters an Omega Metroid, which proceeds to destroy one of the SA-X. Samus absorbs the nucleus of the SA-X and uses her newly-restored Ice Beam to destroy the Omega Metroid, before escaping to her ship, which extracts her from the space station before it collides with SR388.