Three years into the Great War, England’s greatest asset is their intelligence network—field agents risking their lives to gather information, and codebreakers able to crack every German telegram. Margot De Wilde thrives in the environment of the secretive Room 40, where she spends her days deciphering intercepted messages. But when her world is turned upside down by an unexpected loss, for the first time in her life numbers aren’t enough. Drake Elton returns wounded from the field, followed by an enemy that just won’t give up. He’s smitten quickly by the too-intelligent Margot, but how to convince a girl who lives entirely in her mind that sometimes life’s answers lie in the heart? Amidst biological warfare, encrypted letters, and a German spy who wants to destroy not just them, but others they love, Margot and Drake will have to work together to save them all from the very secrets that brought them together.

All of England thinks Major Phillip Camden a monster—a man who deliberately caused the deaths of his squadron. But he would have preferred to die that day with his men rather than be recruited to the Admiralty's codebreaking division. The threats he receives daily are no great surprise and, in his opinion, well deserved. As nurse Arabelle Denler observes the so-dubbed "Black Heart," she sees something far different: a hurting man desperate for mercy. And when their families and paths twist together unexpectedly, she realizes she has a role to play in his healing—and some of her own to do so as well. With Camden's court-marital looming, an old acquaintance shows up, intent on using him in a plot that sends the codebreakers of Room 40 into a frenzy. With their fragile hopes for the future in the cross hairs, Arabelle and Camden must hold on to hope—and to each other—if they want to survive.

Zivon Marin was one of Russia's top cryptographers until the October Revolution tore apart his world. Forced to flee to England after speaking out against Lenin, Zivon is driven by a growing anger and determined to offer his services to the Brits. But never far from his mind is his brother, who Zivon fears died in the train crash that separated them. Lily Blackwell sees the world best through the lens of a camera and possesses unsurpassed skill when it comes to retouching and re-creating photographs. With her father's connections in propaganda, she's recruited to the intelligence division, even though her mother would disapprove if she ever found out. After Captain Blackwell invites Zivon to dinner one evening, a friendship blooms between him and Lily that soon takes over their hearts. But both have secrets they're unwilling to share, and neither is entirely sure they can trust the other. When Zivon's loyalties are called into question, proving him honest is about more than one couple's future dreams—it becomes a matter of ending the war.