1950's San Francisco: Jean (Julie Graham), Millie (Rachael Stirling), Iris (Crystal Balint) and Hailey (Chanelle Peloso), four remarkable women who worked as code-breakers during WWII, use their exceptional decryption skills to solve crimes overlooked by the police. A bloodied and battered stranger collapses on Millie and Jean's doorstep, dragging the code-breakers into a world of clandestine factions designed to conceal illegal motives. As the mysterious attacks escalate the women race to identify the men responsible in an attempt to end the violence. When a Russian diplomat dies in Iris's arms, the FBI sweeps in and rules his death accidental. But with the dead man's last plea ringing in her ears, Iris knows there is more to the story. The women quickly find themselves embroiled in a dangerous KGB plot. Will they find the strength to outsmart their darkest, most sinister nemesis yet?