After their search for the missing Great Priestess Lys leads Rin and Aghyr into trouble and an unexpected battle at the Water Sanctuary, they take some time to regroup among their friends of the Kaishin Clan. There, they meet up with the Fire Oracles Kara and Koru, who have prophesied the arrival of the Scarlet Soul— none other than Rin Shirano herself.

With the Fire Oracles' help, Rin reclaims her sister's sword Hitaken, which has chosen her as its wielder. Whether she feels ready to take on the responsibility or not, she is the only one who can carry the ancient exorcist blade now. With demon activity increasing throughout the land, it's more important than ever that the Great Priestess is found as quickly as possible... even as forces gather that will stop at nothing to make sure Rin can never see her sister again!