Set during World War II, the novel follows two women whose lives intersect through the clandestine power of the printed word: Ava Harper, an American librarian recruited as an intelligence operative in neutral Lisbon, Portugal, and Hélène Bélanger, a Frenchwoman who joins the Resistance in occupied Lyon, France.
In April 1943, Ava works in the Rare Book Room at the Library of Congress when she is recruited by the Office of Strategic Services (OSS), a wartime intelligence agency. Her fluency in French and German and her microfilm experience make her ideal for the Interdepartmental Committee for the Acquisition of Foreign Publications (IDC), a covert program that gathers intelligence from foreign texts in neutral countries. Ava's motivation is personal: Her older brother, Daniel, serves in the Army's 101st Airborne Division, and she feels responsible for his enlistment because he sacrificed his college savings to raise her after their parents died in a plane crash. Despite a fear of flying rooted in that trauma, she boards a plane to Lisbon.
In Lyon, Hélène's husband, Joseph, a former chemistry professor and pacifist veteran of the Great War, has been missing for two days after a bitter argument. A Jewish woman named Claudine Goldstein arrives seeking identity papers, and Hélène impulsively gives Claudine her own card and ration coupons. Now without papers, Hélène turns to Joseph's closest friend, Etienne, who provides a false identity card under the name Elaine Rousseau. Etienne reveals that Joseph is secretly a Resistance operative who forged identity cards and trained guerrilla fighters, and that he has been arrested and held at Montluc Prison. Hélène demands to join the Resistance, and Etienne agrees. She adopts the name Elaine.
Ava's arrival in Lisbon is difficult. Her boss, Mr. Sims, is dismissive, having expected a male agent. On her first morning, Ava unwittingly flirts with a man named Lukas at a newspaper kiosk. A British Allied intelligence operative in Lisbon named James MacKinnon intervenes, warning her that Lukas is likely a Nazi spy. Ava settles into her daily routine of collecting foreign publications and photographing them onto microfilm for shipment to Washington.
Elaine trains with a cell of Resistance women: Nicole, a confident blonde who uses charm as a weapon; Denise, a stern communist whose Jewish husband hides with the Maquis, rural guerrilla fighters; and Josette, an increasingly anxious young woman. They teach Elaine poem-based coding, typing messages on silk, and distributing copies of
Combat, a clandestine Resistance newspaper. Nicole discovers that Joseph was arrested by Kommandeur Werner, a Gestapo officer infamous for cruelty. Etienne later tells Elaine that Joseph has been transferred to a work camp for political prisoners. Elaine writes a desperate note of apology and love and forces Etienne to promise to deliver it. She is reassigned to a secret printing warehouse, where she learns to operate the Minerva, a manual printing press.
In Lisbon, James introduces Ava to Lamant, a French refugee who supplies smuggled copies of
Combat. Through a refugee aid worker, Ava meets Otto Müller, a German-Jewish engineer who entrusts her with a letter from his sister, written during the 1942 Paris roundup of Jews. Mr. Sims berates Ava for wasting microfilm on personal correspondence, but she argues that "this present we live in is tomorrow's history" (187) and wins the right to continue.
One night, a Jewish woman named Sarah and her young son Noah break into the warehouse, having fled a hiding place discovered by the Gestapo. Sarah's husband, Lewis, is already in America. Elaine arranges shelter for them and, when the press leader Marcel refuses her plan to encode a plea for help in
Combat, defies him by secretly embedding a coded message: "Jewish mother and child need transport to America" (199). She repeats the code in subsequent issues until Radio Londres, the BBC's French-language broadcast, confirms London has received it.
In Lisbon, Ava notices unusual typos in an otherwise immaculate edition of
Combat and suspects a hidden code. James recognizes the Resistance's poem-code system, and together they decode the message. Ava persuades James to leverage British intelligence contacts, and England agrees to coordinate a rescue with the Maquis. Meanwhile, Otto takes his own life after being denied an American visa for the third time, deepening Ava's resolve.
Ava and James grow closer, sharing a first kiss while evading the PVDE, Portugal's secret police, in Lisbon's Alfama district. At a dinner party, however, Ava overhears James sharing what sounds like Allied secrets with a man wearing swastika cufflinks. James claims he was feeding misinformation but refuses to elaborate, then vanishes on a two-month mission.
Etienne delivers the devastating news that Joseph has died. Elaine channels her grief into printing and encoding messages. She is arrested by the Gestapo while delivering newspapers and endures a night in Montluc Prison, experiencing the horrors Joseph faced. Brought before Werner, she maintains her cover as a housewife until Nicole arrives, charms an officer, and secures Elaine's release.
James returns to Lisbon with Sarah and Noah, gaunt and limping from a bullet wound sustained in Toulouse. The journey through occupied France was harrowing: Forged papers failed to arrive, and the group crossed the snow-covered Pyrenees on foot. Ava settles them in an apartment and begins battling bureaucracy for American visas. James then reveals the full truth: He is not the librarian he claimed to be but a British special operative who deliberately let the Nazi spy Lukas, whose real name is Dieter Hoffman, believe Ava was his intelligence contact, endangering her to protect his real source. Devastated, Ava orders him to leave.
In Lyon, Radio Londres broadcasts coded signals indicating the Allied invasion is imminent. Nicole fails to appear for a scheduled pickup, and her body is found in the Rhône, bearing evidence of torture. The Gestapo and the Milice, a French collaborationist militia, storm the warehouse. Marcel and Elaine flee but are trapped. Knowing she cannot withstand torture without endangering others, Elaine asks Marcel to shoot her. He fires, but the bullet passes through her torso, missing vital organs. Marcel then takes his own life. Elaine survives and escapes with Etienne's help. Nicole, it emerges, never broke under torture; the informant was her replacement courier, Celine.
On D-Day, Ava learns Daniel's unit is on the beaches of Normandy but eventually receives word: "I'm safe. I love you" (350). After months of setbacks, she secures passage for Sarah and Noah on the SS
Drottningholm and sees them off at the docks. Two months after D-Day, the Allies liberate Lyon. Etienne kills Werner as the Germans flee, and Elaine uses the Minerva to print lists of concentration camp survivors. On Ava's last morning in Lisbon, James confesses genuine feelings, explaining that rescuing Sarah and Noah was something he did for her. They agree to meet in London, where Ava plans to work at the London Library.
In the epilogue, a camp survivor named Saul returns Elaine's note, confirming Joseph carried it in his shoe and was clutching it when he died, shot by a guard while smuggling food to a fellow prisoner. Claudine returns Hélène's original identity card, and Elaine reclaims her birth name, Bélanger. Daniel returns safely, and Ava, realizing she has outgrown her old life, travels to Lyon before departing for London. She finds Elaine at her press and delivers a photograph of Sarah, Lewis, and Noah reunited in America. The two women embrace, connected by a coded message that saved a family and by their shared faith in the power of the written word.