A simple afternoon spent modifying a dress at Carousel Boutique leads to two ponies accidentally discovering that they have more in common than was once believed.

Princess Peach Toadstool of the Mushroom Kingdom has come to Equestria on important diplomatic business, bringing with her the superstars of the Mushroom Kingdom, Mario and Luigi. But, when Discord is released, and aids Bowser's forces in invading Equestria and kidnapping Peach, Celestia, and Luna, Twilight and her friends must work together with Mario and Luigi on an adventure spanning two kingdoms, to rescue the princesses, defeat Bowser and Discord, and return Equestria and the Mushroom Kingdom to their former glory.