After suffering severe floods in 1753, the people of Senbon Matsubara in Shizuoka cooperate on flood prevention around the Kiso, Nagara, and Yuhi rivers. The people of Satsuma (Kumamoto Prefecture) endure great hardships after being forced to undertake the work by the central government, a subtle indication of the bad feeling that would foment into open rebellion the following century.

Legend has it that the human race is not the only dominant civilization living on Earth. Two other races exist in this world: the Makai (a demon race) and the Jujinkai (a half-man, half-beast race). Once every 3,000 years, a supreme being known as the Choujin (Overfiend) will emerge and bring balance to all three realms on Earth. In present-day Japan, after 300 years of endless searching, a Jujinkai named Amano Jyaku has discovered the presence of the Choujin inside high school slacker Tatsuo Nagumo. But now, Amano, along with his sister Megumi and their sidekick Kuroko, must protect Nagumo and his new girlfriend Akemi Ito from the Makai, who believe that Nagumo is not the Choujin, but an evil entity bent on destroying all living beings on Earth.