Set in 1952, "The Bletchley Circle" tells of four women who have returned to normal lives after working at Bletchley Park -- the site where the Germans' infamous "enigma cypher" was broken by British mathematicians and code breakers during World War II. One of them, Susan, notices a pattern to the murders of several women -- which is in the news -- but no one one else seems to have noticed the pattern. She speaks with the police about it but is not very successful, so she enlists the help of her former friends and fellow cryptographers to investigate the cases.