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.