Wario couldn't believe it! It had been such a lovely day, with not a cloud in the sky--a perfect day to fly his plane. Then, without warning, his engine began to sputter, and Wario crash-landed in the woods! He spent
the rest of his afternoon wandering amongst the trees and underbrush,
muttering to himself about his miserable luck. Without looking where he was going, Wario stumbled right into a cave!
Inside the cave, he found a beautiful music box, but no matter how
much Wario wound it up, no song could be heard. "What's-a this?!? It must-a
be brok..." But before he could finish, Wario vanished with a flash! Only the music box remained, right where Wario found it.
Wario soon awoke to very different surroundings, and it wasn't long
before he realized he was inside the music box! A mysterious figure informed
Wario that he had once ruled the world inside the music box, until an evil
being sealed away his magical powers in five music boxes.
Naturally, Wario wasn't too interested in this story... that is, until the figure promised to send Wario back to his own world and let him keep any Treasure he found, if he'd only recover the hidden figure's powers. Enticed by the thought of returning to his own world with a cache of Treasure, Wario departed on his quest-- in search of the music boxes and the many Treasures of this mysterious land!
--from the Game Boy Color Wario Land 3 instruction manual