Whether a Metroid novice or a seasoned veteran, you should play this game
Chronologically, Fusion is the last Metroid, taking place some time after Super Metroid. With a search team deep in the depths of SR388, the Metroids' home world and location of Metroid II, Samus is attacked by a strange jelly-like creature known as X. X is actually a parasite and almost kills Samus from the inside out. Her Power Suit gets corrupted and has to be removed surgically, but even then she has a minimal chance of survival. But then, someone finds a cure. As it turns out, Metroids were the main predators of X, and a Metroid cell culture (made from the Metroid Samus saved in Metroid II and which saved her in Super Metroid) is used to make an anti-X vaccine, which works on Samus instantaneously and essentially changes Samus's cellular makeup. This is where the game gets its name; Samus has fused with the Metroid. Back from the brink of death, Samus is contracted to investigate an explosion at the Biologic Space Labs where her team sent all of the specimens from SR388, and this is where the game begins.
As I have learned since, Fusion deviates from the classic Metroid formula in the sense that the game doesn't let you wander around and explore on your own. From the outset, Samus is given orders from an A.I. that she names Adam that point the player in a more-or-less straight line throughout the game. This formula may turn off more classic players, but it also makes Fusion the perfect "beginner's Metroid" because of the features and quirks of the series it brings without the worry of getting lost and frustrated.
For example, the is one part in Sector 4 of the space station that requires Samus to acquire the Speed Booster to break through a wall so she can lower the water level. While in say, Super Metroid, if you are unable to find the Speed Booster after a while, you'll begin to wonder whether or not you're actually in the right place, and all too often this leads to the player leaving the area they're in to search somewhere else, never knowing how close they were because eventually, the frustration will win out and the game will never be touched again. However, in Metroid Fusion, Adam doesn't let Samus out of the area she's in until she's accomplished the task he's set forth for her, so that's one off the list; you know you're in the right area.
Even so, the several sectors are all huge, and it's very easy to get lost in them. This leads to learning another Metroid staple: shoot and bomb everything, because there's always something hidden, whether it be a very important passage, a column to stand on or just another missile tank. The areas (or "sectors" as they're referred to in-game) are all self-contained with maps Adam gives you, but there are so many hidden and unmapped portions that learning to find the hidden wall or one breakable block is just as important this time as it has been in adventures past.
Metroid Fusion also contains some very high quality cinematics for a Game Boy Advance game, which lead to it being the most story-driven game yet in the Metroid series. The opening of the game recounts Samus's near-death experience with many very sharp still-frames and a few animations that help draw the player in like few games of this size have done before. Once the story of the SA-X gets going, you will actually finds yourself with chills a few times thanks to the quality artwork, great graphics and superb music, especially with the music. More than once did my spine tingle with fear when I found the SA-X chasing me to a very panicky theme, or when the music comes to halt and it is too quiet; much like in old movies, this elevates the feeling of tension fivefold.
After Fusion, I went on to play Zero Mission, finish Prime and Echoes and finally play Metroid and Super Metroid and love them all to death, but despite it's difference I always remember and occasionally come back to the one that started it all. The redeeming value that led me to go with Fusion in the first place was its unlockables with Metroid Prime, and given the frustrations I had with Prime at the time, it's quite possible that if I had never played Metroid Fusion, I wouldn't have ever played a Metroid game I liked ever again. Play Metroid Fusion. You won't be sorry you did.