Taken from this Months Nintendo Power.

“Every hundred years a comet passes over the Mushroom Kingdom and rains down magical stars and stardust, and as Princess Peach and the citizens of the kingdom are celebrating the centennial event, Bowser and his legions attack. As Mario attempts to put a stop to Bowser’s shenanigans, but things go awry: the next thing he knows, he finds himself on a mysterious moon high above the Mushroom Kingdom.”

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