The codebreaking work at Bletchley Park was supported by an extensive infrastructure of outstations, the largest being at Eastcote, which later became GCHQ. By consulting archival documents, visiting the wartime bases and talking to those with personal knowledge, the author has pieced together the stories of these lesser-known sites. The book also features analysis of the improvements to Alan Turing’s Bombe machine and highlights the vital contribution of the Wrens in operating this equipment.