World War Two reshaped naval warfare, with secret weapons playing pivotal roles. This episode explores these transformative innovations. We examine advances in submarine warfare, from the Japanese I400 submarines to the German Type XXI U-boats, along with groundbreaking systems like the URSEL Rocket U-boats and the UGS Manta, a craft merging hydrofoil speed with midget submarine stealth. Engineering marvels, from Mulberry Harbours to Yamato-class battleships, show nations' quests for an edge. We also touch on visionary concepts, like an aircraft carrier crafted from Pykrete.