Gaiking: Legend of Daiku-Maryu

A boy, Tsuwabaki Daiya found a black fire in an ocean when he and his father were in a small boat. A big monster came out of the fire, and the father and the crews were disappeared into the sea. When he was about to become the next victim, a large mecka dragon appeared and saved him.<br><br> 5 years later since then, Daiya becomes a junior high school student. A big monster reappear, and he meets the mecha dragon,"Daiku Maryu". The head of the dragon separates from the body, and it turns into the body of a super robot, Gaiking. He gets in Gaiking, fights against the monster, and beats it.<br><br> He learns that there is an enemy that plots to conquer the world, and he leaves for another world, Darius, to stop the plot. <br><br> (Source: AnimeNfo)