Mario defeats Bowser, saves Peach, and then goes back to the Mushroom Kingdom. Back at Bowser's castle, Kamek is working on a special potion. He tests the potion out on a Goomba, and the Goomba grows twice its size. Kamek decides to show the potion to Bowser. Bowser uses the potion to send the new gigantic enemies to the Mushroom Kingdom. The Toad Brigade notice the huge enemies and decide to try and stop them.
Dreams are needed to place Toads while Mushrooms are needed to place other characters.