Follow four ordinary women with the 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, Susan, Millie, Lucy and Jean have returned to civilian life, keeping their intelligence work secret from all, including family and friends. A series of ghastly murders targeting women, however, re-unites the team as they set out to decode the pattern behind the crimes.

Part 1 of 3
Susan, noticing a pattern in a string of London murders, contacts her old friends from Bletchley. They treat the crimes like a code to be cracked and try to identify the killer.