Ten years ago, the Underground Empire was led by Dr. Hell, a malevolent scientist, in a plot to wipe out all humans. A young hero named Kouji Kabuto fought against Dr. Hell's evil scheme with the help of the renowned super robot, "Mazinger Z," and emerged victorious, bringing peace back to the planet. Since then, Kouji has left his pilot position and pursued a career in the field of science, carrying on his family's legacy. However, his fate takes an unexpected turn when a chance encounter deep in Mt. Fuji presents an even more lethal threat to humanity. Now tasked with deciding the future of mankind, Kouji and his robotic partner must choose between aligning with good or giving in to evil.