Official story and features:
"Venture to a fantastic island filled with magical monsters in a far distant world. The 'Black Evil' has taken over this lush and colorful land and captured the once-happy Maboo tribe natives. Drawing on your imagination and endless possibilities, you must create your own monster and enter it into nearly 30 different competitions involving timing, speed, and head-to-head battles against the 'Black Evil' to win back the island and banish the evildoers!"
Story from Sega site
"- Patented Monster Editing System creates an untold number of monsters, each with distinctive features and abilities
- With each successful competition, unlock new features for monster enhancement and possession of one of eight magic orbs
- Transfer your monster to Game Boy Advance to take on three of your friends or play alone and acquire special GBA-only monster-building items"
Features from Sega site