Long ago there was a go-kart war to get a legendary kart for racing, in the end the winning side decided to destroy it, turning it into seven pieces that only a clash between light and dark could summon
Modern day Mushroom Kingdom, Peach is unveiling a new theme park based around go-karting, when suddenly a mysterious cloaked figure appears, takes over the theme park and it tells Mario and friends that if they want their theme park back they must first beat his trials.
Each World is a zone of the theme park, taken over by villain of the Mario series and turned into a death trap more reminiscent of their character. At the end one of the characters must fight the character that took over the zone, Mario Vs. Wario, Luigi Vs. Waluigi, Peach Vs. Bowser, DK Vs. K. Rool, etc.
At the end of the game the mysterious hooded figure reveals that the whole reason he made the heroes fight the bosses was to cause a clash between light and dark, which would summon a piece of the legendary go-kart. He teleports each of the hero's piece of the kart into his hand, and pieces them together to summon the legendary go-kart. He then challenges the heroes to a race, but before the race starts he takes of his cloak and reveals himself to be Bluster Kong, he then transforms into Leo Luster and then one of the playable characters must race him.
In the end the heroes win, the theme park is returned to normal, and the power of friendship turns out to be more powerful then a mysterious go-kart.