Gaiking: Legend of Daiku-Maryu

One day, while out on a small boat with his father, a boy named Tsuwabaki Daiya discovers a strange black fire in the ocean. A massive monster emerges from the flames, dragging his father and the crew into the depths. Just as Daiya is about to meet the same fate, a giant mechanical dragon appears and rescues him. Five years later, now a junior high school student, Daiya encounters the monster again-along with the mecha dragon, "Daiku Maryu." The dragon’s head detaches and transforms into the powerful super robot Gaiking. Daiya pilots Gaiking, defeats the monster, and learns of a greater threat: an enemy plotting to conquer the world. To stop them, he sets off for another world, Darius.