The following version of this novel was used in the creation of this study guide: Quinn, Kate. The Rose Code. HarperCollins Publishers. 2021. Paperback.

In 1947 after World War II, Osla Kendall receives a cypher from Clockwell Sanitarium where her former friend Beth Finch is incarcerated. Beth claims that there is a traitor who had her locked up when she discovered the truth. Osla contacts Mab who refuses to speak with her.

Shifting back to 1940, Mab, her mother, and her sister Lucy all live in London as war is on the horizon. Meanwhile, Osla meets Prince Philip of Greece and falls in love with him. Osla and Mab are sent to Bletchley Park where people work on decoding German messages that have been encrypted by the Enigma machine. They are told not to reveal anything about their work. The two are sent to live at the house of Beth Finch who Osla helps get a job at Bletchley Park under Dilly Knox. After Beth helps crack the Italian Enigma, she stands up to her abusive and overbearing mother and chooses to leave the Finch family home to focus on herself.

Over the next few years, Mab meets and marries a poet named Francis Gray who she met early on in her time at Bletchley Park. She later reveals to him that Lucy is actually her daughter, something he guessed already when they first met. The two plan to move to Coventry together once the war is over.

Meanwhile, Osla is rescued from an air raid by a man named J.P.E.C Cornwell and tries to contact him via letters. Then, she is forced to give up her relationship with Philip to protect him from being accused of treason. Beth also becomes romantically involved with Harry, who is only married to take care of his son, Christopher. Shiela, Harry’s wife, encourages the relationship, but Beth is devastated when Harry decides to enlist in the army so that his son no longer has to be bullied because his father is not a soldier.

Along the way, Beth solves a code that lists Coventry as the next target of attack, but does not say anything because she is not allowed to share secrets. Osla and Mab travel to Coventry with Lucy and Francis but they are separated when Osla accidentally lets go of Lucy’s hand. They are reunited after the sirens stop, but their house collapses, killing Francis and Lucy. Mab blames Osla, destroying their friendship.

Meanwhile, Beth decides to crack the Soviet Enigma which Dilly was working on before he died. She finally does so in June of 1944, naming it the Rose Code. However, she learns that there is a traitor at Bletchley Park. Before she can reveal this, she is locked away in the Clockwell Sanitarium for a supposed mental breakdown after Mab and Osla, who learn that Beth did not inform them of the Coventry raid, do not stand up for her.

Three years later, Beth learns that the traitor is actually a friend named Giles Talbot who tells Beth to give him all the evidence of his treason. When she refuses, he tells her that she will receive a lobotomy the day before the royal wedding if she does not comply. A week before this happens, Osla and Mab arrive and realize that Beth is telling the truth and Giles, who is now Osla’s fiancé, is the traitor. They help her escape and know they only have one week to decipher the rest of the Rose Code and prove his crime. Mab then calls Harry who never visited Beth because her mother told him she was dead. Thanks to Harry, they are able to access a bombe machine and Enigma machine and Mab learns that her new husband is actually a bombe technician, but neither were allowed to tell each other the truth behind their pasts.

With the Enigma cracked, the women are able to capture Giles, but everyone is arrested. Since Osla is still wearing the coat of Mr. Cornwell that he gave to her in the raid near the beginning of the story, he is called in and the two finally meet. Then, Osla calls Prince Philip to help. Afterward, Giles is sent to another institution while Beth is freed and continues her relationship with Harry. Mab and her husband develop a closer relationship, while Osla marries Mr. Cornwell and becomes a writer for the magazine called The Tatler. In the end, Bletchley Park is opened as a tourist destination in 2014, and all three women refuse to give up their wartime secrets.