While Bowser is "training" in his castle by breaking bricks, he hears a loud "thud". Believing it to be Sumo Bros, he investigates, but is surprised to find that a group of Toads has broken into his castle. The Toads explain that while Bowser was "training", they had completely taken over the Koopa Kingdom.
Enraged, Bowser defeats the group of Toads with his new shell dash technique and sets off to take back his kingdom, planning to seek revenge against the Mushroom Kingdom when he is finished.