The story takes place in a quasi-medieval period. It tells of two twins who were separated shortly after they were born. The reason was that one of them (Rin), was chosen to be the next Diva, and her destiny is to sing for eternity to keep an ancient Dragon in its slumber.
After Rin's brother (Len) was old enough, he began searching for his sister, to free her from her fate of being a Diva.