When middle school student Jun Tendou is chased by a strange creature into a temple, he is shocked when the head priest, Genun, offers more details about the oni demon he had just evaded. Before he has time to consider this new information, Genun suddenly transports them both back in time to the Heian period, where Heian-kyo—the ancient city which would eventually become modern Kyoto—is facing regular attacks by these same oni.

The nobles of Heian-kyo see Jun as their savior; as the descendant of a special bloodline, Jun can control the dragon Orochi and use its tremendous force to defeat the oni. However, not only is Jun hesitant to resort to violence, but he also discovers that the oni are not what they appear to be.