When her brother Hiroki disappears after a violent altercation with bullies, Yuriko finds a magical book in his room. She learns that Hiroki has been possessed by The Book of Heroes, and that only she can save him. With the help of the monk Sky, the dictionary-mouse Aju, and the mysterious Man of Ash, Yuriko has to solve the mystery of her vanished brother and save the world from the evil King in Yellow.