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. 
(Source: themanime.org)

Nazi Germany - 1944: As American bombers soar towards Berlin, a revolting Satanic rite takes place below - Hitler and his minions are using the monstrous Nazi Death Rape Machine to render the dimensional boundaries separating the Human World from the World of Man-Beasts and the World of Monster Demons. Dr. Munhihausen Sr.`s life work is about to come to fruition - the conjuring of the Overfiend! 
Cut to modern day Tokyo - Munhihausen Jr. searches for a monster with strength enough to conquer the omnipotent Overfiend. Since legend says that he who kills the Overfiend will rule all three worlds forever, Munhihausen is determined to slay the Overfiend and assume complete control over all three dimensions. His efforts seem fruitless - until Nagumo`s cousin Takeaki, infused with Nagumo`s blood after an accident, begins to develop monstrous powers. 
As Man-Beast Amano Jyaku and his sister Megumi, together with faithful accomplice Kuroko attempt to thwart Munhihausen`s evil plan, nothing seems likely to prevent Munhihausen, descendant of a race of brutal tyrants, from enslaving the world under his despicable rule!

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