When he was little, Ryuuta was attacked by a youkai. An older boy came to his rescue, he was a wandering youkai exorcist called Izumo. Soon after the incident Izumo departed on his travels again, leaving Ryuuta behind, but promising to come back one day and start their own journey together. Years later they meet again, but Izumo seems to have no recollection of their promise. Nevertheless, Ryuuta is determined to stay with him, and they end up travelling together.
(Source: MangaUpdates, edited)