When a UFO from the Vega Alliance arrives on Earth, humanity faces its biggest threat in an oncoming Alien invasion. But their hope arrives in another Alien, Duke Fleed, a refugee of the planet Fleed who finds his home in Japan. Duke pilots the mighty Grendizer, a weaponized Mech to fight off the Vega forces. Along with his best friend and partner Koji Kabuto, they fight to free Earth from the terror of the Vega forces.