(Spin-off of The Bletchley Circle) Jean and Millie depart London for San Francisco to investigate the murder of a close friend. There they are joined by North American code-breakers Iris and Hailey who, like their British counterparts, find themselves undervalued and overlooked despite their indispensable contributions to the war effort. With renewed purpose, the code-breaking team will stay in San Francisco and continue to solve mysteries together in the Bay Area.