The man who fell into the desert has no memory. Koji Kabuto picked him up and named him Daisuke. Koji has two faces: a billionaire and a champion of justice, thanks to his wealth made from the power of photons, a clean energy source. Although Daisuke has a somewhat shady side, he possesses extraordinary talent, and Koji begins to trust him. Just as it seemed that peaceful days would continue, unidentified aerial phenomena began to appear over major cities around the world. It is the Vega Star Alliance Army, attacking from a star system far away from Earth. Humanity is trampled by Vega, whose weapons far surpass any on Earth. Mazinger Z, the iron fortress that rises from the water, is Earth's last hope as it is forced into a difficult battle against the Vega Star Alliance Army. In the midst of this dire situation, the alien demon Grendizer appears, kicking up a cloud of sand. Its pilot is Daisuke, whose real identity is a prince named Duke Fleed who had been exiled from his home planet.
—Anime News Network