Based on the true story of ‘Englandspiel’, one woman must race against the clock to uncover a traitor, even if it means losing the man she loves. England, 1943: Deciding to throw herself into war work, Nancy Callaghan joins the Special Operations Executive in Baker Street. There, she begins solving ‘indecipherables’ – scrambled messages from agents in the field. Then Nancy meets Tom Lockwood, a quiet genius when it comes to coding. Together they come up with the idea of printing codes on silk, so agents can hide them in their clothing to avoid detection by the enemy. Nancy and Tom grow close, and soon she is hopelessly in love. But there is a traitor in Baker Street, and suspicions turn towards Tom. When Nancy is asked to spy on Tom, she must make the ultimate sacrifice and complete a near-impossible mission. Could the man she loves be the enemy?