Mario and Luigi are on a mission to rescue the seven kings of the seven worlds which neighbor the Mushroom Kingdom.
Each of the kings has had his magic wand stolen by one of the Koopa Kids, who has turned him into a different type of animal. Its up to Mario and Luigi to enter the seven worlds, make their way to the Koopa Kids airships and take back the wands.
However, as Mario and Luigi learn at the end of world 7, these attacks are merely a diversion planned by Bowser, to get Mario and Luigi out of the way while he kidnaps Princess Toadstool.
So its up to Mario and Luigi to save the Princess once again!