On November 8, 1947, in London, 26-year-old Osla Kendall wonders what to wear to the upcoming royal wedding between Prince Philip, her ex-boyfriend, and Princess Elizabeth. The mail arrives, including a coded message from Bethan “Beth” Finch, an inmate at Clockwell Sanitarium. Beth and Osla worked at Bletchley Park’s top-secret codebreaking operation during World War II, forming a friendship together with Mabel “Mab” Churt, though the three of them later became estranged.

Osla decrypts the message, then phones Mab. Mab confirms that she received a similar message but refuses Osla’s request to meet. Within the psychiatric hospital, Beth hopes Osla and Mab will believe her messages.

In December 1939, 21-year-old Mab lives in a small East London apartment with her mother and her four-year-old daughter, Lucy, who passes as Mab’s sister. In her spare time, Mab reads library copies of literary classics “for the Well-Read Lady” and dreams of providing a better life for Lucy (13), whether through hard work or an advantageous marriage.

Also in December 1939, Osla returns to England after a brief stay with relatives in Canada. Upset at Osla’s return, her mother temporarily cuts off her allowance.