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, 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…

Original cast members Rachael Stirling and Julie Graham are back as formidable team Millie and Jean.

“The Bletchley Circle: San Francisco” begins on ITV on Wednesday 25th July at 9pm.