The Metroid series started in 1986 on the 8-Bit NES system. There was no other game like this back when it came out. The gameplay relied solely on exploration and discovering the secrets of the planet Zebes. Perhaps it was the most difficult in the series, due to the lack of a map or any hint of direction. Then in 1992, Metroid II came out for the original GameBoy. The graphic style was enhanced and there were new elements and a new planet, named SR388. The goal was to clear out all Metroids and restore peace in the galaxy. Metroids were organic creatures with an unknown orgin. They thrived off the energy and life consumed from it's prey. Thus, this is why Metroids were eliminated. The game still suffered the same problem as the 86 Metroid, no map.
Then, in 1994, one of the greatest games ever made was released, Super Metroid. It was perhaps the most perfect edition of Metroid available. It had the classic gameplay, updated graphics, a highly explorable world, big bosses, plenty of weapons and items, and finally, A MAP! You can see where you go and where you have been. It's not to say that I didn't get stuck, I did, but the map really helps. I can't think of any other game, aside from Castlevania: Circle Of The Moon, that had the in-depth, compelling gameplay Super Metroid had.
Enter Metroid Fusion. The 4th platform Metroid in the series picks up after Super Metroid left off. On a routine mission, Samus Aran returns to SR388 due to suspicious activity. During the trip, she is attacked and rather consumed by a parasite named "X". Near death, she is taken to a space lab and given a vaccine from the last "remaining" Metroid. This recovers her fully, but totally reconfigures her DNA. She can't use her old suit any more due to the X parasite, so a new one had to be fashioned. Because of this, all her abillities must be re-learned and reconfigured. She can't use an Ice Beam since ice freezes Metroids (and she has the Metroid DNA). But she can now consume the X parasites, since that's what Metroids like to consume (and the reason for the exlosion of the parasites on the planet).
But there's a problem. Abord the station, the X parasites have been released and are consuming the creatures on the ship. Composed of 6 specialized environments, such as fire, ice and darkness, Samus must find all of the X and eliminate them. But it's not easy. Samus's old suit has been taken over by X, and is now a new being. It looks like Samus, fights like Samus, but is after you and sabatouging the ship. Why? Play the game.
Graphically, the game looks good, but not as good as Super Metroid. I blame it on the art style here. In Super Metroid, the game looked grittier and more meanacing. In Metroid Fusion, it has some of that, but then there's areas that look slightly comical. A little smooth and pastel in some backgrounds, but then other areas get it right. The game as a whole looks great. It never slows down, and throws out some neat effects, such as when the X parasites form, and the use of some scaling. The animations are well done as well. It doesn't look as choppy as Castlevania : COTM, or as small.
The sound, again is not as good as Super Metroid, but for a GBA game, it's pretty good. The music is moody and thematic and does match the style of Super Metroid, but it doesn't come out with as much drama as the SNES game. Again, it's the limitation of the GBA. It can only do so much.
The game really excells in gameplay. Even though the setting is different, and even slightly odd, but it does feel like Super Metroid (so many refrences!). But, Metroid Fusion pushes you into a more specific direction via a computer Samus calls Adam. When you report to the station, you're given an objective. This is not like the Metroids of the past. You were ususally just left to fend for yourself and find out where to go next. Now, you're told what to do. It almost seems linear, but later on, things loosen up. And even though you're told what to do, it's not obvious in which path to take. It also gets more difficult as you go along. You will have to find alternate routes and you will run in circles at times. That is what Metroid is about. Overall, Metroid Fusion excells at gameplay and delivers what longtime fans want.