The invaders came without warning, conquering Earth and turning many, including Kento, into an orphan. The year is 1995, and Kento and his friends now live a destitute life, stealing food and steering clear of the mysterious alien tower that looms near their village. But when the gang happens upon an underground cave, they discover Doctor Earl's secret base, futuristic technology, and even a giant mecha! Kento must take his place in the cockpit and defend the planet from the evil Akron race - but little does he know the truth about his heritage, and what that means to the war.
Years ago, after watching his family die at the hands of the Vegan empire, Duke Fleed managed to escape, stealing the powerful UFO Grendizer that the enemy used to destroy his planet in the process. Eventually ending up on Earth, the boy was adopted by Dr. Umon and given the name Daisuke, intent on living out the rest of his life in peace. However, when his father and the mecha pilot Kouji Kabuto realize that the Vegan empire has arrived at Earth to repeat history, Daisuke is forced back into the pilot's seat to save his new home. Alongside Kouji's flying saucer, can Daisuke and his Grendizer stop the fleet and save the world?