Bounty hunter Samus Aran explores the surface of the planet SR388

Bounty hunter Samus Aran explores the surface of the planet SR388 with a survey crew from Biologic Space Laboratories (BSL).

There, she is attacked by parasitic organisms known as "X".

Samus is unknowingly infected by the parasite, and proceeds to return to the BSL station,

when she suddenly loses consciousness and 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 made from a remaining cell culture of the infant metroid fusion Fight that Samus had previously contacted,

as the Metroids were the X's main predators on SR388.

The vaccine cures Samus and while it gives her the ability to absorb and use the nuclei of X Parasites for nourishment,

it also burdens her with the Metroids' vulnerability to cold.[11] Her infected Power Suit is sent to the BSL station for examination,

although parts of the suit were too integrated with her body to safely remove during surgery, forming a partially armored suit called the Fusion Suit.

When Samus recovers consciousness, she discovers that an explosion has occurred on board the BSL station.

She is sent to investigate. The mission is overseen by her new gunship's computer whom Samus nicknames "Adam", after her former commanding officer, Adam Malkovich.

Soon after her arrival, Samus learns that the X parasites can replicate their hosts' physical appearances and memories, killing them afterward.

She also discovers that the X have infected the station with the help of the "SA-X", an X parasite mimicking Samus at full power.

Samus narrowly avoids the SA-X on several occasions as she continues to explore the space station,

107 completing various objectives such as saving the sentient Etecoons and Dachoras still on board the ship.

By using Data Rooms and defeating larger creatures infected by the X, Samus begins to recover her various abilities.

She later discovers a restricted lab containing Metroids.

Suddenly, the SA-X appears, and in an attempt to destroy the Metroids sets off the restricted labs' auto-destruct sequence.

Samus escapes but the lab is destroyed. Discovering this, the computer berates Samus for ignoring her orders,

and admits that the Federation was secretly using the lab to breed Metroids.

It also reveals that the SA-X managed to asexually reproduce before perishing, subsequently cloning itself;

in light of this, the computer advises Samus to leave the station.

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