After the fairies defeated Gunala, the Evil Goddess, another crisis hit the Fairy Castle!! The magical energy of the world has been devoured by the evil wizards. The Rainbow Flower, the source of the magic power, is soon to wither, and this will destroy the world of Fairy Castle. Only finding the legendary Rainbow Heart Stone and letting it recollect all the essential energy can save the world. To solve this biggest crisis in the history of the Fairy Castle, Prince YouLe, Sally, and their new friend XiaoQian, have to go back to the human world and reunite with Maggie and Michelle to fight the evil again.

(Source: Mahou Shoujo Wikia, edited)