Mario and Luigi: The Mushroom Kingdom Turned Upside Down is a fictional Mario and Luigi game. It is released for the DSi system.
The game begins when Mario and Luigi are watching a parade in Toad Town with Peach and Yoshi. Unfortunately, Bowser appears at the end of the parade and kidnaps the Princess, forcing Mario and Luigi to go after him. After a brief battle, Bowser ends up falling into toxic acid, and becomes Dark Bowser. Dark Bowser takes off, and the Mario Bros. take off after him.
The two eventually come to Sunnyside Resort and meet Alissa the Rabbit, a rabbit who is crying because her sand castle was washed away by the tide. She gives the Mario Bros. under a minute to rebuild it. Once they do rebuild it, she blows them a kiss and gives them a Star Bit, a mysterious object that gives the Bros. strength. Alissa asks the Bros. to visit her father, Razmanian Metal in Kalamari Desert.
Mario and Luigi reach Kalamari Desert, where they climb aboard the Kalamari Desert Express, the train that runs through the Desert. Once aboard, they meet Razmanian Metal himself, the engineer of the train. Unfortunately, he won't give up his Star Bit without a fight. After a brief fight, the Bros. claim their Star Bit, just as Toad calls them to come back to Princess Peach's Castle, and hurry!
Once there, Toad explains that Dark Bowser is causing havoc throughout Mushroom City, and they better stop him. And so, Mario, Luigi, and Yoshi set off to Mushroom City. Once again, the trio encounters Dark Bowser, who bombards them with Bob-ombs. Yoshi eats the Bob-ombs and spits them back at him, causing Dark Bowser to retreat to Dark Bowser's Keep. The Bros. and Yoshi follow him there.
At last, the three friends reach Dark Bowser's Keep, and yet again encounter Alissa, who is crying because Dark Bowser came out of nowhere and scared him. The Mario Bros. leave Yoshi with Alissa, and go off to fight Dark Bowser.
After a hard and long battle, Mario is attacked by Dark Bowser. Mario thinks it's over for him, but then, Alissa turns off the security system that keeps Dark Bowser alive, rendering him dead.
And so, another party is held at Sunnyside Resort, with Dark Bowser turned back to normal Bowser.