The 'black evil' has taken over the island, and the Maboo Tribe have called upon you to bring order and outcast the evil that has taken control. To do so, you must create your own creature and train it up, entering it into various competitions against the 'black evil' and working towards the main goal. As the competitions are completed, you will unlock new features to improve your own creature in speed, strength and general looks.
The main reason to do so are the mini games which the monsters can participate in. While those rely on your active gamepad skills (mostly pressing a button as fast as possible with minor variations), the changes of success are dictated by the monster's statistics. So for example a dragon would be beneficial for a flying mini game. Those statistics can be further enhanced with items bought at shops.
Play single and multiplayer battles on the GameCube, or upload your creatures to the Game Boy Advance, taking them on the road to challenge your friends and unlocking unique items and features for the creatures you design.