(Part 1/2). 1942, London, England. Code-breaker Claire Hopkins, on the eve of being transferred to Bristol after purposely disposing of an American communication, is found murdered with her tongue cut out and a mysterious symbol drawn on her hand. Fast forward fourteen years, and former colleagues Millie and Jean learn of the murders of two women in San Francisco in identical circumstances. Jean and Millie travel to San Francisco to try to make contact with a former American codebreaker, 'Major Sixth', whom Jean had been in regular communication with during the war. Jean learns that 'Major Sixth' is in fact Iris Bearden, wife of honoured US Armed Forces sergeant Marcus Bearden. Iris introduces Jean and Millie to Hailey Yarner, one of the youngest codebreakers at the time of the Americans joining the war. As the four women meet to try and work out the killer's pattern, a third victim is discovered and Iris begins to fear for her son, Dennis, a civil rights activist, who is part of a group opposing property developers bull-dozing their district, Fillmore - which just happens to be the killer's next target.

(Part 2/2). With San Francisco in the grip of a serial killer, the four allies work together to expose a sinister conspiracy at the highest levels - and in the process, forge a powerful bond.

(Part 1/2). Newly-weds driving on a country road find a body, apparently the victim of a hit and run. She was the unhappy wife living the country club life. Iris, Millie, Jean and Hailey investigate the discovery of an apparent code book, and discover a nest of lies and deceit.

(Part 2/2). The apparent code book isn't what it seems to be, but it still provides clues. Detective Bryce makes an arrest and Millie has a date.

(Part 1/2). When a bloodied and bruised stranger collapses on Jean and Millie's doorstep, they find themselves drawn into a complex maze of problems involving a gang of violent drug dealers and a secret group of 'homophile' men communicating through the newspaper small ads. But as they try to figure out how the two organisations could be connected, Millie's cousin Edward becomes the next victim. Millie asks Detective Bryce to meet with Edward and hear his story, and Edward reveals his homosexuality to Bryce. After he's released from the hospital, Edward is accused of being a Communist and arrested; leading Jean and Millie to believe that Bryce had betrayed their trust.

(Part 2/2). Unsure of who they can really trust, Jean and Millie are shocked to discover they are facing deportation as a result of their association with Edward. They suspect that Detective Bryce was instrumental in ensuring their fate. Hailey and Iris find out the name of the party placing the cryptic ads, and Hailey convinces her former boss to place an ad in the newspaper in an attempt to flush out the dealers. As Jean and Millie stake-out the drug dealers' secret hideout, they are shocked to witness Detective Bryce liaising with their leader. The group finds out that Edward used to be a member of the Mattachine Society, and Millie discerns that he and the stranger who was attacked had been more than just friends. The refusal by the leader of the Mattachine Society to help Edward angers Hailey and she reveals her homosexuality to Iris. The circle convinces Rusty to help them, and the group and the drug dealers finally meet in a showdown at the Big Bop club.

(Part 1/2). Iris and Hayley's attendance at a soirée involving a world-renowned cellist ends in unexpected disaster when a Russian diplomat collapses and dies right in front of Iris' eyes. Meanwhile, Jean is pleased to discover than an old acquaintance from England, Nigel Beamish, is in San Francisco working for the British consulate. But after meeting Nigel for a romantic afternoon picnic, Jean discovers she has picked up the attention of some unwanted admirers.

(Part 2/2). Iris, Hailey and Millie ask Olivia for help cracking the cipher which could potentially reveal where the Russians are holding Jean. Hailey realizes she has been betrayed and watches on in horror as her cipher machine disappears in the arms of the mysterious Ilyia. Iris is forced to choose between her head and her heart when the girls track the Russians to a private airfield and find that Deborah has secretly been in league with them all along.