High school student Youko Nakajima is confronted by a stranger named Keiki, who swears loyalty to her and drags her into a fantasy realm called the Twelve Kingdoms as demonic creatures attack. Stranded and marked as a kaikyaku, an outsider from another world, she is hunted through unfamiliar lands, forced to survive with only a sword that chooses its wielder. Youko must learn the kingdoms' laws and the weight of the Mandate of Heaven before she can find her place, or a way home.