It's 1956, two years after we last saw our women, when Jean is confronted with evidence that the killer who struck down a Bletchley girl during the war… has resurfaced in San Francisco. With a scrap of war-time code from an American counterpart (a brilliant cryptographer who worked in the Presidio) Jean and Millie set sail to San Francisco to locate some of those code-breakers, who they hope might be able to lend support to their investigations. Decoding the decade-old teletype message leads them to an underground jazz club in the heart of San Francisco, where they meet Iris — an American musician and math genius who served at the Presidio during the war. She, along with her young engineering prodigy Hailey, is convinced to join forces with their British visitors, in order to track a series of murders that have shattered the Fillmore District — a vibrant downtown neighbourhood where Iris happens to live. His twisted signature: cutting out victims' tongues and drawing an inexplicable box on their palms.