A captivating story presented in game form.
The game starts w/ a cutscene explaining the present and peculiar situation of its main character, Samus. Apparently, Samus has been running around and killing the titular metroids for the past few years and this has had some unexpected consequences. Specifically, the metroids, which are these weird jellyfish creatures that are apparently very bad, were the exclusive predator of the X parasite. X parasites are even worse than metroids, if you can believe that. They can assume the form of literally any living creature and what's worse they've infected our dear Samus. Fortunately, she was saved by a vaccination created from the last remaining metroids. She was severely weakened in the process but she is now immune to the X parasite and that makes her a reasonable choice to entirely eradicate the rest of them--xenocide being these people's only means of dealing w/ extraterrestrial life, apparently.
This bit of context is huge. The gamer is put right there w/ Samus, fighting for her very existence, alone on a space station w/ only her commanding officer, an AI construct, to keep her company. It is a compelling story and sets the foreboding atmosphere perfectly. I found myself continually looking forward to the next cut scene or piece of information that furthers the plot.
That the gameplay was good didn't hurt either. It revolves around Samus running around blasting enemies and collecting new abilities and power-ups. The enemies can be quite powerful, esp. when you first arrive in a new area and a few of the boss fights were kind of ridiculous--though there are frequently tricks to make them much easier. Samus also has quite a lot of exploring to do. As she gains new abilities, she can access new areas and find additional power-ups. Some necessary locations are quite well hidden off the beat path and can be a trial to find. The gamer playing through the first time, will undoubtedly wind up wandering some of the same territory over, re-fighting the same enemies sometime ad nauseum. It is frustrating but ultimately fulfilling once the proper path is found.
Because the plot is so central, Samus is more or less stuck on one course of action. She can go basically anywhere in the station during the game but, for the most part, she must focus on the task at hand, especially towards the end of the game. Still, because of the many optional power ups, Metroid: Fusion has a lot of replay value. Going for 100% powerups is fun but its real potential is in speedruns and seeing how many of the powerups you can skip and still beat the final bosses. The linear structure of the game only gets slightly grating during the last little bit of playtime, where it feels as if Samus is just marching into boss fight after boss fight w/ little in between. It works well overall though and the game does a good job of balancing storytelling and gameplay.
Actual in-game time to complete the story takes about a half dozen hours give or take on the first run-through, though most will spend another few hours on it due to dying periodically. It may be short by today's standards but it is dense and very little of the game feels superfluous, only very rarely becoming a grind. This is a great game for those who enjoy the dark atmosphere of Metroid games but desire a little more structure in their gaming experiences. It shows off wonderfully how well the Game Boy Advance can do a fully engaging adventure-style game.