Long ago, an elderly couple in the remote mountain village of Nagakushi were entrusted with the care of two infants — a boy and a girl — as foretold in a dream. The children grew up believing themselves to be the couple's own. When they were eight years old, an evil dragon appeared in the western capital and began turning humans into oni — creatures that consumed the souls of others. The oni swept toward Nagakushi and stole the souls of the couple who had raised them. Left unharmed by whatever grace protected them, the two children set out to save their adopted parents — not yet knowing that the journey would reveal where they actually came from.