Deep within Wario's castle lies a treasure room filled with the spoils he has plundered during his many adventures. A mysterious black jewel, ensconced among the treasures, has a strange power to transform gems into monsters. While Wario rests in his castle upstairs the jewel begins to morph his treasure trove into a host of monsters, turning the basement of Wario's beloved castle into a bizarre parallel universe. Awakening the next morning to monsters instead of treasure, Wario must step into that alternate world to win the treasure back.