When WWII breaks out, an elite force of German U-Boat commanders, under the direct command of Vice-Admiral Karl Dönitz, attempt to starve Britain into submission by sinking hundreds of thousands of tonnes of their merchant shipping in the mid-Atlantic. Vera Laughton Mathews heads the newly reformed WRENS, and recruits bright, astute, and mathematically minded women into the service. Meanwhile retired naval commander Gilbert Roberts, who has been out of the service, is keen to return to his career and use wargaming to sink the U-Boats.
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.
When Gilbert Roberts arrives to set up his proposed Western Approaches Tactical Unit (WATU) for U-Boat wargaming, he receives a decidedly frosty reception from Admiral Sir Percy Noble who is convinced the Allies already possess the tactics and expertise to defeat the Kriegsmarine. Nevertheless, Roberts presses on and, enlisting the help of Jean Laidlaw and a small team of WRENS, based in Liverpool, begins the process of round-the-clock wargaming to devise tactics to outmanoeuvre the U-Boat attack formations.
oberts 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.
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.
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.
