It is 1st January 1942. U-boat attacks have reduced Britain's supply line from the United States to a trickle. If Britain runs out of supplies, it will lose the war.
Churchill thumps on the table and demands that the Royal Navy determine why Hitler's U boats are winning the Atlantic war. The Royal Navy recalls an officer, Gilbert Roberts, who had left the force due to injury. He is brought back because he has a unique talent: he knows how to recruit and train war gamers. With a dedicated team, he can unpick the German U-boat strategy and work out how to defeat the opposing force.
But there's one problem: there are no male officers available to join a war gaming school. They are all at sea. Captain Roberts, undaunted, recruits a team of women - all members of the Women's Royal Naval Service (WRENS). Most are in their twenties. He finds a mathematical prodigy, an accountant, and even a top-class hockey player. None of the women has ever been to sea. Together these Wrens unpick how the U-boats are sinking so many British and American ships, and they even devise tactics to defeat them.
On 5th May 1943 their tactics are used against the largest fleet of German U-boats ever to attack a convoy. The convoy is ONS.5. In a massive battle, which stretches from Iceland far out into the mid-Atlantic, the U-boats are frustrated and defeated by the WRNS' war gaming.
The Germans call the month of that battle 'Black May'. In 31 days, they lose 41 U boats. In the end Grand Admiral Doenitz reluctantly withdraws his U-boats from the Atlantic.
But the Wrens' story has never been told - until now.