Gather the Cruiser Parts!
From the Super Mario Wiki
Prior to the start of the level, Captain Goomba, Captain Shy Guy, Captain Boo, and Captain Koopa Troopa run into Fawful, who claims that "Bowser is not a thing that is not of this world" and that he and Bowletta had taken over Bowser's Castle. The Goomba Squad encounters the damaged Koopa Cruiser, which they decide to rebuild. They battle Clumphs, Ice Bros, and Chuck Guys, but do not find anything yet. Meanwhile, Fawful recruits Roy to stop the Goomba Squad.