Planet Earth is not all it seems. It is revealed that humans are not alone and that there are unseen realms running parallel to our own: the realms of the Demons (Makai) and the Man-Beasts (Jyujinkai). To further that, there is a 3000-year-old legend that foretells the coming of the Overfiend (Chōjin)—a being of unimaginable power that will unite all three realms into a land of eternity.

The story follows the exploits of the protagonists—man-beast Jyaku Amano, his nymphomaniac sister Megumi and their companion Kuroko—as their 300-year search for the Overfiend takes them to a high school in Osaka, Japan. Their discoveries led them to two students: shy, lecherous Tatsuo Nagumo and school idol Akemi Ito. But as the film plunges deeper into the dark and gruesome, Jyaku discovers that the Legend of the Overfiend is not what it seems and that the future of the three realms may be strikingly different from what he is led to believe.