Duke Fleed is a survivor from Fleed Planet who escaped to Earth aboard a mysterious "spaceship". There, he was raised by Dr. Umon as his adoptive son and known as Daisuke Umon. Years after his arrival, he faces the threat of King Vega and his army, who want to conquer Earth. Along with his friends Koji and Hikaru (and later his younger sister Maria Grace), Duke decides to fight back using his best weapon - the almighty Grendizer.