Tsuwabaki Daiya is out on a small boat with his father when a black fire appears in the ocean. A huge monster rises from the flames, and his father and the crew disappear into the sea. Just as Daiya is about to be taken too, a giant mechanical dragon arrives and rescues him. Five years later Daiya is a junior high student when a monster strikes again, and he meets the same dragon, Daiku-Maryu. Its head separates from its body and becomes the body of a super robot, Gaiking. Daiya climbs into Gaiking and defeats the monster. He then learns that an enemy is plotting to conquer the world, and he sets out for another world, Darius, to put a stop to the scheme. The result is a mecha action series with a sci-fi bent.