In December 1968, Thomas Harding, an elderly, terminally ill man in Richmond, London, prepares for a final trip to Paris. He carries a bundle of wartime letters and one sealed envelope marked "To be opened in Paris on Christmas Eve" (8), fulfilling the dying wish of the woman he loved. On the plane, he unties the ribbon around the first packet and begins to read, carrying the reader back to the outbreak of World War I.
Thomas, a scholarly young man at Oxford, enlists in the British Army alongside his best friend, Will Elliott, hoping that military service will mend his rift with his father, who wants Thomas to run the family newspaper, the
London Daily Times (LDT). Will's sister, Evelyn (Evie) Elliott, begins writing to both men, establishing a playful correspondence with Thomas that gradually deepens. The friends plan to celebrate Christmas together in Paris, but the war drags on and the trip is postponed. Evie chafes at being left behind and searches for ways to contribute. Alice Cuthbert, Evie's closest friend, shares her restlessness and her quiet fondness for Will.
Thomas and Will ship out to France in November 1914. Thomas describes the realities of trench warfare with increasing honesty, including a Christmas Day truce in which British and German soldiers sing carols together in no man's land. When Charlie Gilbert, a soldier Evie's mother favors as a suitor, is killed in action that December, the danger facing Thomas and Will becomes painfully real.
Through 1915, Thomas and Evie's correspondence intensifies. They exchange literary references, particularly Shakespeare and William Blake, and develop a coded "Language of Stamps" on their envelopes, their intimacy growing beneath friendly banter. Thomas's father falls seriously ill with cancer, and Thomas reluctantly asks his cousin, John Hopper, to oversee the LDT despite his father's distrust of the Hopper family. Hopper suggests that Evie write a column offering a woman's perspective on the war, and Evie begins writing a column titled "A Woman's War" under the pen name Genevieve Wren, quickly building a devoted readership.
In May 1915, Will is mortally wounded in a mortar attack. Thomas is by his side when Will dies, and the loss devastates Evie's family and shatters Thomas, who falls into heavy drinking before pulling himself back. Evie returns to her postal duties and writing, finding purpose in both. Thomas comes home on leave in October, and he and Evie spend every day together. Evie confides to Alice afterward that she may be falling in love, though neither she nor Thomas has spoken of romantic feelings. Thomas's father, weakened by cancer and injuries sustained in a zeppelin raid on London, dies on December 1, 1915; in a final dictated letter, the elder Harding tells Thomas he was always proud of him. On Christmas Day, Evie writes a love letter to Thomas but lacks the courage to send it.
The war's toll on Thomas reaches a breaking point in March 1916 when a grenade lands in his trench. He hesitates, and five men, including his commanding officer, are killed. Consumed by guilt, Thomas falls into a severe emotional collapse and goes silent for months. Evie writes letter after letter without reply. Alice, now a Voluntary Aid Detachment (VAD) nurse in France, eventually locates him: He is alive but experiencing war neuroses, a form of psychological trauma then called shell shock, and has been transferred to Craiglockhart War Hospital for Officers in Edinburgh, Scotland. In desperation, Evie sends the unsent love letter, reasoning that the fear of never seeing Thomas again outweighs the fear of rejection.
Evie travels to Scotland and visits Thomas regularly at the hospital. They meet on a bench beneath an oak tree, and Thomas confides in her about the grenade incident and his shame. Evie is patient but does not press her romantic feelings, sensing he is too fragile. He recovers enough to return to France, placed under the care of a nurse named Rose Blythe.
While home that Christmas, Evie discovers hidden letters in her mother's desk between Will and a French nurse named Amandine Morel. The letters suggest a secret relationship and hint at a possible child. Will had written to their mother asking her to help Amandine, but Mrs. Elliott concealed the correspondence. Evie writes to Amandine but receives no reply.
Hopper proposes to Evie on Valentine's Day 1917, but she delays her answer. Instead, she enrolls in the Women's Army Auxiliary Corps (WAAC), a military service branch for women in noncombatant roles, and ships out to France as a telephonist, writing Hopper that she will find her answer overseas.
Thomas is furious when he learns Evie has come to France. Evie fires back, quoting Charlotte Brontë's
Jane Eyre: "I am no bird; and no net ensnares me." Thomas apologizes, clarifying he only wants to protect her. From France, Evie's columns grow bolder and more critical of wartime propaganda, drawing acclaim and scrutiny from the War Office.
The breaking point comes in mid-1918 when Hopper writes to Thomas claiming he and Evie are planning a spring wedding. Evie has never accepted, but Thomas believes the lie and severs their friendship. The War Office shuts down the LDT after Hopper prints Evie's most provocative column against her wishes. Evie formally rejects Hopper and writes Thomas a passionate confession: "I love you, Thomas Harding [...] I love you, and always have, and always will" (351). He refuses to respond.
Meanwhile, Amandine's mother, Sophie Morel, replies to Evie's letters, revealing that Amandine died in childbirth and that her daughter, Delphine, survived and lives with Sophie in Paris.
In October 1918, Evie falls gravely ill with the Spanish Flu. Rose sends Thomas a telegram: Evie is asking for him. At the same time, Charles Abshire, the LDT's bookkeeper, forwards Evie's Christmas 1915 love letter, lost in the mail for three years. Reading it, Thomas's anger dissolves. He races to Rouen, but a quarantine bars him from the hospital. He writes Evie confessing he has loved her since childhood, "since the first time you threw rocks at the pigeons on my lawn" (363), and begs her to fight. The Armistice is declared on November 11, 1918, but the letters do not reveal whether Evie survives.
During the 1968 Paris interludes woven between the wartime sections, Thomas visits the Tomb of the Unknown Soldier at the Arc de Triomphe, leaving cufflinks Evie had commissioned bearing portraits of Will's horses. He meets Delphine, now a grown woman with her father's blue eyes, and gives her a gold wren necklace that had belonged to his mother and later to Evie, along with Will's letters and an unsent letter expressing Will's joy at learning of Amandine's pregnancy.
On Christmas Eve 1968, Thomas opens Evie's sealed final letter. She recalls waking from her illness to find Thomas at her bedside, confirming she survived and that he broke through the quarantine with Rose's help. She reminds him of Christmas 1919, when he proposed atop the Eiffel Tower and she said yes before he finished asking. She tells him not to fear death: "We are but birds in flight, you and me. Let us catch the thermals together now, and soar" (369). Thomas whispers a Christmas greeting to Evie and closes his eyes as a carol drifts up from the street, implying his peaceful death.
In an epilogue set in February 1969, Delphine writes to Will Harding, Tom and Evie's son and the current editor of the LDT. She reports that Thomas was buried beside Evie in Richmond and suggests publishing the wartime letters as a historical series. She invites the family to visit Paris in the spring, bringing the long-deferred dream of celebrating together in Paris full circle at last.