During World War 2, the Nazis attempted to breach the barriers between the three worlds. They failed then, but a Dr. Myunihausen (Munchausen?) tries again, through the manipulation of Nagumo’s brother Takeaki. His goal? To have the Chojin replaced.
As Takeaki is injured in the approaching flight, Nagumo donates some of his blood to save his life, thus infusing Takeaki with the powers of the overfiend. And the results might be too much for Tokyo, perhaps even the overfiend himself, to bear.
Prequel: Choujin Densetsu Urotsukidouji
Full story: Shin Choujin Densetsu Urotsukidouji Mataiden
Sequel: Choujin Densetsu Urotsukidouji: Mirai-hen