Kamek mistakenly transforms the koopa king into Bowsette, but surprisingly she succeeds at what the koopa king has been trying to do for years, capturing Mario and taking over the Mushroom Kingdom! Perhaps worse, she's trying to grow as huge as she can with all the mushrooms in the kingdom, so nobody can ever surpass her.
Bowsette has grown into a giantess, but it's still not enough. She's on a search for more power, and has found something. At the heart of an ancient temple lies a black tar said to possess immeasurable power, and Bowsette still needs more. She still needs to get even bigger.