With an extraordinary flair for code breaking and razor-sharp intelligence skills, four seemingly ordinary women become the unlikely investigators of a string of grisly murders in this original thriller, set against the backdrop of post-war London. Susan (Anna Maxwell Martin, Bleak House), Millie (Rachael Stirling, Women in Love), Lucy (Sophie Rundle, Episodes), and Jean (Julie Graham, William & Mary) worked together at top-security HQ Bletchley Park to help crack German military codes, producing crucial intelligence that shortened the war. Now in 1952, they have returned to post-war domesticity, and although Susan may appear a typical wife and mother, when she hears about a string of unsolved murders in London, Susan's old Bletchley spirit is ignited. She reconvenes the formidable foursome to solve the murders - bohemian and streetwise Millie; Lucy, with a brilliant photographic memory, and Jean, the methodical no-nonsense organizer. Can they rekindle their expertise and discover the killer's next move - before the killer strikes again?