After the fall of France in 1940, Dönitz's trio of Kriegsmarine U-Boat aces embark on a race to send British merchant ships to the bottom of the ocean, converging on merchant convoys in co-ordinated wolfpack attacks. In Britain, an increasingly desperate Admiralty sends for Roberts, at last willing to entertain his newfangled wargaming ideas to try and identify ways of thwarting the deadly U-Boat wolfpacks.

Roberts and Laidlaw have uncovered a fatal flaw in the Royal Navy's existing anti-U-Boat tactic ‘Buttercup'. By wargaming different scenarios, they develop a countertactic of their own, codenamed ‘Raspberry', in which a merchant convoy escorted by Royal Navy destroyers can lure in the wolfpack before surrounding and destroying it; in effect, playing the enemy at its own game. But the team now faces an uphill battle to try to convince Noble and his colleagues at the Admiralty that they hold the key to stopping the U-Boats in the Atlantic.