Following the destruction of Mother Brain on Zebes intergalactic bounty hunter Samus Aran was returning back to HQ when she lost consciousness and her ship crashed into an asteroid. Scientists recovered her escape pod and brought her back to HQ where it was determined that a parasite has invaded her nervous system during her previous adventure on Zebes. Dubbed the X Parasite, the bio-organism began to bond to Samus and her power suit. The scientists were forced to amputate and remove key componants of the suit and arranged to have them transported for further study. Samus, however, was near death even still. At the last crucial moment it was discovered that a vaccine made from the last remaining (although now deceased*) Metroid could save her life. It seems that the Metroids were the natural predators of the X Parasite, and now with them extinct the Xs had free reign of the planet.

Samus awoke in the lab a shell of her former self and her appearance forever altered. The portions of her suit that had been removed were the parts that enabled her to do her amazing moves and attacks. With them gone, she was only capable of operating at 10% power. Normally she would have been allowed to recover, but somebody sabotaged sectors of HQ and was allowing the X Parasite to spread. Samus, equipped with a new ship, was allowed to investigate provided she take it easy on herself.

And so begins the fourth Metroid game in the series, 2002's Metroid Fusion, for the Game Boy Advance. Nintendo has totally overhauled Samus Aran, giving her a new power suit, new ship, and new abilities. She retains her basic moves from Super Metroid and picks up a few new ones, such as grabbing ledges and climbing ladders. One of the biggest changes to Samus is that due to her parasitic infection she is able to absorb energy from stray X Parasites. As the game progresses and Samus absorbs more and more parasites, she is able to perform some of her old trademark moves, such as the morph ball.

Samus' primary mission is to stop the X Parasites from taking over the station, however she must also determine who is allowing the parasites to take over and put a stop to him/her. As she travels around the station she ventures into simulations of old areas, such as a replica of SR388. Each area is connected by a central hub, the HQ, and is accessible via elevators. Each area contains several different rooms that are devoted to saving the game, refilling energy, obtaining a new ability, or downloading new maps and receving mission updates.

Metroid Fusion features the ability to be connected to its Nintendo GameCube cousin, Metroid Prime which unlocks a few hidden goodies. Completing MF and connecting the games unlocks the original Metroid game in all its 8-bit glory, while completing MP and connecting the games allows the Samus in MP to sport MF's new power suit. Reaching out even further, the game can also link up to 2004's Metroid: Zero Mission in order to unlock some hidden character artwork.

This entry in the Metroid saga is pure gold and it is currently available for sale at your favorite local video game story. A sequel/prequel to the game, Metroid: Zero Mission, was released in February 2004.

* As seen in Super Metroid


References:
Various articles on nintendo.com and n-philes.com