Mario and his friends arrive in Delfino Isle on vacation. However, the people on the island imprison Mario. Someone who looks like him has been making the island dirty. A judge tells Mario to clean up the island. He is given a device called F.L.U.D.D. (Flash Liquidizer Ultra Dousing Device), which Mario straps on his back. F.L.U.D.D. is a powerful water device that can clean up the mess that the fake Mario has caused. Mario learns that Bowser Jr., Bowser's son, was the one who changed the island and kidnapped Princess Peach. Mario sets out to find them.