Duke Fleed is a survivor from Fleed Planet who has fled to Earth. Aboard a mysterious "spaceship," he is raised by Dr. Umon as his adopted son and becomes known as Daisuke Umon. Years after his arrival, he faces the threat of King Vega and his army, who seek to conquer Earth. Alongside his friends Koji and Hikaru, and later his younger sister Maria Grace, Duke decides to fight back using his most powerful weapon - the almighty Grendizer.