Spring 1946. Ten months after the end of World War II an explosion rocks the Pacific off the coast of Hawaii. America has just destroyed one of Japan's most advanced weapons systems, the I-401 aircraft carrier submarine. But this was no belated attack against the defeated Japanese. Bound by an agreement to share any discoveries with the Soviets but feeling the pressure of the looming cold war it was a calculated decision to keep the technology out of Soviet hands. The Japanese had built their sub to take the war to U.S. shores. Merging the stealth and tactical advantages of sea and sky the revolutionary submarine carried three specially designed Seiran attack bombers which could be launched from the deck of the sub within seven minutes of it's reaching the surface. With missions to attack U.S. cities and blow up the Panama Canal the aircraft carrier submarine had the potential to change the course of the war in the Pacific. But fortunately for America it's secret weapon - the atom bomb - was put into action first. "Japanese SuperSub" investigates Japan's efforts to take submarine technology where it had never gone before and reveals how close the Japanese came to using the sub for an attack on the U.S.