According to ancient lore, humans are not the sole advanced civilization inhabiting Earth. Alongside them are two other races: the Makai, a demonic species, and the Juujinkai, beings that are half-human and half-beast. Every 3,000 years, a powerful entity called the “Choujin” (or Overfiend) is destined to appear and harmonize these three worlds. In contemporary Japan, after centuries of relentless searching, a Juujinkai named Amano Jyaku has identified the Choujin within a high school student, Tatsuo Nagumo. Now, Amano, his sister Megumi, and their companion Kuroko must safeguard Nagumo and his new girlfriend, Akemi Ito, from the Makai. The Makai are convinced that Nagumo is not the Choujin but rather a malevolent force intent on annihilating all life on Earth.