Suddenly, a UAP (Unidentified Aerial Phenomenon) of the Vega Alliance appeared on Earth. Humanity is helpless in the face of Vega's overwhelming military might, but then "Mazinger Z" operated by Kouji Kabuto, and "Grendizer" piloted by Duke Fleed, appear on the scene. Why did Duke Fleed, the prince of the planet Fleed, come to earth? What kind of life did he spend in his homeland? The story of "The Inception" opens with a prequel that is not told in the TV anime