Zatch Bell! 101st Devil

During the Summer holidays, Gash and gang decide to head for Fuji Mountain for a picnic gathering. There, they encounter a girl with a white magical book. Her name is Kotoha and her book has a message saying that Gash's mother is located a cave in the forest. However, when they eventually found the cave, there was already a blond-haired youth by the name of Wiseman. In order to rescue Wiseman, Gash and the others attempt to proceed into the depths of the cave and stumble upon the entrance to another world. Soon later, the strongest warrior, the Black Knight, appeared before them. Thinking that Gash was the one who stole the 101th magical book, the Black Knight started to attack them. Gash and Kiyomaro have to find the real criminal in exactly 24 hours, or else they will be stuck in the alternative world forever.

Synopsis from Anime News Network