Lt. Mellie Blake is looking forward to beginning her training as a flight nurse. She is not looking forward to writing a letter to a man she’s never met - even if it is anonymous and part of a morale-building program. Lt. Tom MacGilliver, an officer stationed in North Africa, welcomes the idea of an anonymous correspondence - he’s been trying to escape his infamous name for years. As their letters crisscross the Atlantic, Tom and Mellie develop a unique friendship despite not knowing the other’s true identity. When both are transferred to Algeria, the two are poised to meet face-to-face for the first time.

It is 1941, and America teeters on the brink of war. Outgoing naval officer Ensign Jim Avery escorts British convoys across the North Atlantic in a brand-new destroyer, the USS Atwood. Back on shore, Boston Navy Yard secretary Mary Stirling does her work quietly and efficiently, happy to be out of the limelight.

Never pretty enough to please her gorgeous mother, Allie will do anything to gain her approval - even marry a man she doesn't love. Lt. Walter Novak - fearless in the cockpit but hopeless with women - takes his last furlough at home in California before being shipped overseas. Walt and Allie meet at a wedding and their love of music draws them together, prompting them to begin a correspondence that will change their lives. As letters fly between Walt's muddy bomber base in England and Allie's mansion in an orange grove, their friendship binds them together.

When German bombs set London ablaze, BBC radio correspondent Hugh Collingwood reports on the Blitz, eager to boost morale while walking the fine line between truth and censorship. But the Germans are not the only ones Londoners have to fear as a series of murders flame up amid the ashes. The deaths hit close to home for Hugh, and Aleida needs his help to locate her missing son. As they work together, they grow closer and closer, both to each other and the answers they seek. But with bombs falling and continued killings, they may be running out of time.

When the Germans march into Denmark, Baron Henrik Ahlefeldt exchanges his nobility for anonymity, assuming a new identity so he can secretly row messages for the Danish Resistance across the waters to Sweden. American physicist Dr. Else Jensen refuses to leave Copenhagen and abandon her research-her life's dream. While printing resistance newspapers, she hears stories of the movement's legendary Havmand-the merman-and wonders if the mysterious and silent shipyard worker living in the same boardinghouse has something to hide.