Captain Yuuki Agamatta was the captain of a Japanese whaling ship, the Shiumi. He and his nephew, Gisei, spotted a herd of sea serpents and caught one of the beasts with a harpoon. When they brought it on board, the mother attacked the Shiumi, killing Gisei and wounding Agamatta. Using his family fortune, Agamatta rebuilt his missing body parts and rebuilt the Shiumi as a robotic mega ship, the Shiumi's Justice. He took off a revenge mission on Umihaha, with only his first mate, Kai Nezumi by his side.