The Bletchley Circle follows Susan, Millie, Lucy and Jean, ordinary women with extraordinary ability to break codes, a skill honed during World War II when they worked undercover at Bletchley Park, site of the United Kingdom's main decryption establishment. Now, in 1952, the four have returned to civilian life, keeping their intelligence work secret from all, including family and friends. A series of ghastly murders targeting women, however, reunites the team as they set out to decode the pattern behind the crimes.