From here:
"Aoi was an ordinary boy until a wizard from the Land of Frogs, Makaeru, splashed magical water over him, turning Aoi into a frog. Makaeru then throws Aoi into a tree trunk, transporting him to the Land of Frogs. He is found by Princess Mimori, Makaeru's sister, and together they go on a quest to find the missing pages of the Magic Book that can remove Aoi's curse. For you see, when Aoi gets wet, he turns into a frog, and when he is dry, he turns back into a human..."