The advanced I-403 Submarine was the summit of Japan's navy capabilities during World War II. The craft was able to exceed depths of nearly 3,000 meters, as well as contain the newest line of guided torpedoes. Truly a sight to behold the ship was, until one fateful day. Patrolling the seas, protecting it's beloved land, the I-403 Submarine was attacked and captured. Fearing that the surface dwellers' fleet would surpass their own, the undersea Mu people seized the vessel. But when they entered the ship, they found no crew, no captain. Only a few scraps of papers that promised the construction of another submarine. Terror now caressed the people of Mu. Captain Shinguji would soon build and show the world of Japan's newest weapon: the undersea warship Gotengo.