The is the possible plot that was revealed:

"Princess Peach is on a vacation on Dolpic Island when many large scribbles appeared on the island. Mario vows to bring those responsible for this crime to justice and clean up the island in the process. Until then, Princess Peach can't finish her vacation! Mario must track down the culprit making use of his new waterpump backpack."

It`s good to see no typical "rescue the princess" storyline, even if this one is a bit odd. Scribbles appear on the island? Mario vows to bring those responsible for this crime to justice? We`ll know more at E3, but Mario Sunshine sure is sounding interesting.

source GameCube Europe