The story of Japan's little-known secret weapon during World War II, an aircraft carrier-submarine that could have launched attacks on the coastal U.S. By the time two were in the water, however, the war's dynamics necessitated recasting their purpose; and before any sort of attack could be take place, the war was over. The episode details the innovative design of the subs as well as the planes they carried, and features remarks from experts and Japanese veterans who served aboard the vessels.