After surviving the destruction of his home planet, Duke Fleed finds refuge on Earth aboard a peculiar 'spaceship,' under the care of Dr. Umon who raises him as Daisuke Umon. As the years pass, Duke, now faced with the menacing King Vega and his forces bent on Earth's domination, joins forces with his companions Koji and Hikaru. With the later addition of his younger sister Maria Grace, Duke resolves to combat the threat using his most powerful asset - the formidable Grendizer.