Suddenly, the Vega Alliance Army's UAP (Unidentified Aerial Phenomenon) appeared on Earth. Humanity was powerless against the overwhelming military force of Vega, but then Koji Kabuto's iron castle, Mazinger Z, and Daisuke Umon (Duke Fleed)'s alien demon, Grendizer, appeared. Why did Duke Fleed, the prince of Planet Fleed, come to Earth? What kind of days did he spend on his home planet? The story of "The Inception" begins with a prequel that was not told in the TV anime!!