The game takes place during the Bohrok War (please don't ask, just search via internet what is all about). In the beginning of the game, you select one Matoran from a list and play as him for the rest of the game. Only one Matoran is available from the game's outset, Kongu, and the rest of the Matoran become playable as the pieces of their masks are collected throughout the courses. You begin with Kongu, a Le-Koro native who looks like a turtle wearing a weird mask, kinda a hockey mask. He must travel through Le-Wahi to find Turaga Matau (who is basically a similar but taller creature) and defeat the Bohrok and infected Rahi he meets along the way. Each of the two characters has different skill sets. Kongu defeats enemies with a Bamboo disk on his back and jumps high, whilst Turaga can bust down walls and stand on switches but with lower jump. At certain points you need to switch between the two in order to solve a (rather simple) puzzle i.e. activating buttons for secret doors or destroying certain walls or floors in order to progress. Of course during your quest you will find several enemies and hazards such as flying insects and hazards like lava pools etc. Health can be restored, bu finding health packs (depicted as hearts) during your quest.
The game is played with the GBA's D-pad, jumping with the A button and attacking with the B button, whilst switching characters is done with the left or right Shoulder buttons. There is a total of 6 worlds to discover and at the end there is always a big boss to defeat too.
Bionicle: Matoran Adventures is basically a simple platformer in which you hit a few switches, hop a few platforms. It's not the best one found for the GBA, nor bad either!
The backgrounds are pretty detailed and so is your character and the rest of the sprites. Sprites are pretty well designed whilst animation is quite impressive and smooth. The environments are well rendered, but the game is rather flat I think and repetitive. There are even some special GFX effects such as sprite scaling, but it's far from enough to give the game an eye-candy factor.
Sound is OK but not enough to electrify you. While music works well enough, the sound effects are just plain annoying.