Following a promotion to Grand Admiral of the Kriegsmarine, reporting directly to Hitler, Karl Dönitz fills the ocean with as many U-Boats as he can lay his hands on. In 1943, as the battle reaches a crisis point for the Allies, and supplies are spread more thinly than ever around the British Isles, pressure mounts on Roberts and the WRENS to prove their worth. When, in May 1943, convoy ONS 5 catches Dönitz's attention, the stage is set for the battle that will turn the tide of the Atlantic war.