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The Stardust Fields is an area that encompasses the border between the Mushroom Kingdom and the Beanbean Kingdom. Many broken stars can be found in the area, as well as military entrenchments, such as sandbags, barrels, cannons, and spikes. The area also holds a few Bike Pads and a Green Pipe.
The passengers of the Koopa Cruiser fell here when Fawful destroyed it. Mario, Luigi, and two of the Bowser baddies fell on the Mushroom Kingdom side, but Bowser himself fell on the Beanbean Kingdom side into Tolstar's cannon. The fate of the rest of the Bowser Baddies remains unknown. Mario and Luigi went into the building marking the official border, and met the Border Bros. They had them play Border Jump before they were allowed to officially cross the border. The Mario bros. were given a free map of the Beanbean Kingdom to commemorate being the first people to cross the border.
Mario and Luigi soon discovered Bowser stuck in a cannon, and met Tolstar, who required them to give all of their coins to him. However, when it was discovered that they held Mushroom Kingdom coins, Tolstar informed them that they had only given him what was worth 10 Beanbean Kingdom coins and to find 100 more coins in the Stardust Fields. While searching for these coins, they met the Starshade Bros. who taught them how to use 2 Bros. Actions (the High Jump and the Spin Jump) and 2 Bros. Attacks (Splash Bros. and Bounce Bros.). Upon returning to Tolstar with 100 coins, he reveals that he isn't going to free Bowser after all, and fights the Mario Bros. He is defeated but then falls in the way of the path forward. However, one of the Starshade Bros. fires Bowser into him, knocking Tolstar out of the way, but sending Bowser flying into Hoohoo Mountain. The Bros. then travel forward, reaching a cave that takes them to Hoohoo Mountain.
After Mario and Luigi return to the main Beanbean continent after traversing through the Oho Ocean Seabed, Fawful with Cackletta's soul in his Vacuum Helmet discovers Bowser unconscious on the Mushroom Kingdom side. Fawful lets out Cackletta's soul, who proceeds to possess Bowser, turning into Bowletta.