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.

WATU's tactics are tested to the limit in the final conflict for ONS 5, with over 50 Allied ships and their escorts facing off against 30 German U-Boats. In the end, it is the allies who win out and the remnant of the U-Boat fleet is left to try and limp back to its base at La Rochelle. As the Battle of the Atlantic reaches its crucial turning point, however, the work of the WATU is not yet over. Dönitz makes one last, desperate throw of the dice, unleashing an advanced torpedo to try and turn the tide back in favour of his remaining U-Boats. Roberts and Laidlaw must do everything in their power to counteract the new threat and recapture the seas in time for the impending Allied landings in Normandy.