January 1943: Admiral Karl Dönitz, head of the Nazis' U-boat fleet, has brought Britain to the brink of starvation by ruthlessly destroying close to a thousand of their merchant ships. If the transatlantic shipping route is cut off, the Allies will lose their last foothold in Western Europe. 

The Royal Navy turns to retired war gamer Gilbert Roberts. Roberts is to use war gaming to try to decipher and combat Dönitz's tactics. To do this, he needs a team, but the Navy can't spare any men. Instead, he risks the ridicule of high command by turning to the Women's Royal Navy Service (WRNS) to war game the U-boats' tactics.

In partnership with Jean Laidlaw, one of Britain's first female chartered accountants, and a small team of resourceful female mathematicians, Roberts acts out naval battles and games the U-boats' moves on a linoleum floor, using chalk and wooden model ships.