German U-Boat commanders under Vice-Admiral Karl Dönitz sink hundreds of thousands of tons of merchant ships in the mid-Atlantic during WWII in an attempt to starve Britain into submission. Vera Laughton Mathews recruits bright, intelligent, and mathematically minded women to join the newly reformed WRENS. Gilbert Roberts, a retired naval commander, is eager to return to his career and sink U-Boats using wargaming. 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 WRENS) 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.