Somewhere in France

Set during the First World War, the novel follows Lady Elizabeth "Lilly" Neville-Ashford, the youngest daughter of the Earl and Countess of Cumberland, as she defies her aristocratic family's expectations to find purpose, independence, and love amid the devastation of the Western Front.

In July 1914, Lilly attends her brother Edward's engagement ball at the family's London residence and reunites with Robert "Robbie" Fraser, Edward's best friend from Oxford and now a general surgeon in London's East End. The son of a Glaswegian dustman, Robbie visited the family estate seven years earlier, when Lilly was nearly thirteen, and does not recognize the woman she has become. On the balcony, he tells her he will join the medical corps if war comes and encourages her to pursue war work, insisting that women in the twentieth century can achieve anything they set their minds to. Lady Cumberland cuts their conversation short. Robbie leaves abruptly after Lilly's mother tells him Lilly is engaged to a young man named Quentin Brooke-Stapleton, a fabrication designed to drive him away.

After war is declared in August 1914, Edward enlists and Robbie, now a captain in the Royal Army Medical Corps (RAMC), is sent to a hospital in Versailles. At Edward's suggestion, Lilly begins corresponding with Robbie. Their letters grow frequent and intimate: He confides his frustration over infections killing soldiers whose wounds should not be fatal; she shares her longing to contribute meaningfully to the war effort.

Lilly's former governess and friend, Charlotte Brown, who has left her post to train as a nurse with the Voluntary Aid Detachment (VAD) in London, pushes Lilly to take action. Lilly persuades her mother to let her stay at the family's Cumbria estate after Christmas, and once free from supervision, she convinces John Pringle, her father's retired chauffeur, to teach her to drive and maintain vehicles, progressing from motorcars to a temperamental lorry.

In March 1915, a local vicar spots Lilly driving the lorry and writes to her parents. Summoned home, Lilly discovers her mother has been intercepting her letters from Robbie. Lady Cumberland demands the correspondence end, denounces Robbie as beneath Lilly's station, and threatens to evict Pringle and his elderly parents from their tied cottage. Lilly argues fiercely but cannot change her mother's mind. She packs her belongings and leaves for Charlotte's lodgings in Camden Town.

On her own for the first time, Lilly rents a room from Charlotte's landlady and sells her inherited jewelry to support the displaced Pringles. The VAD and the First Aid Nursing Yeomanry (FANY), a women's volunteer ambulance corps, both reject her applications for lack of qualifications. The London General Omnibus Company eventually hires her as a painter, and after a year she is promoted to bus conductor, work that is honest but far short of what she hoped to do.

In October 1916, Robbie is granted leave from the 51st Casualty Clearing Station (CCS), a frontline surgical unit in northern France where he now performs trauma surgery. They meet at a tea shop near Victoria Station. Robbie describes the horrors of triage, the unceasing flood of casualties, and is moved to tears. When he mentions the name Quentin, Lilly insists there was never any engagement, and Robbie realizes her mother invented a fiancé to keep them apart. At the station, as he bends to kiss her cheek in farewell, she turns her face and their lips meet.

At Christmas, Edward reveals that a War Office friend has told him about plans for a Women's Army Auxiliary Corps (WAAC) that will need thousands of women, including drivers. When the announcement appears in February 1917, Lilly applies immediately, secures references from Charlotte and Robbie, and is accepted as a Worker in the motor transport division, assigned to train at Shorncliffe Camp in Kent.

At Shorncliffe, Lilly befriends Constance Evans and wins over her roommates Annie Dowd and Bridget Gallagher. When a hostile lorry instructor reduces a trainee to tears, Lilly steps forward and demonstrates her mastery of the vehicle, drawing on everything Pringle taught her. The instructor appoints her the group's teacher, and her willingness to help transforms her standing among the other women.

In late July 1917, Lilly and seven other drivers volunteer for ambulance routes near the Front. Lilly quietly arranges to have herself and her friends posted to the 51st CCS, where Robbie works. When he sees her, he is furious, arguing that the red cross on her ambulance offers no real protection from German shells. Lilly stands her ground, telling him she came to make a difference, just as he once told her she could.

The women begin a grueling routine, driving Ford Model T ambulances over cratered roads between the advanced dressing station and the clearing hospital. Lilly names her ambulance Henrietta. In the evenings, she reads to patients and helps them write letters home. Constrained by fraternization rules, she and Robbie resume their correspondence in secret, slipping letters into her ambulance and onto his desk.

At a ceilidh organized for the camp, Lilly and Robbie dance together for the first time since the engagement ball. Afterward they slip away to the mechanics' garage, where he kisses her properly. Over the following weeks they meet secretly, but Robbie refuses to let their physical relationship progress, insisting on maintaining honor. The tension becomes unbearable, and he eventually insists they stop meeting altogether.

The 51st is shelled, destroying the reception marquee. That night, a dud shell in the compound detonates, knocking Lilly to the ground. In the aftermath, Robbie demands she transfer to a safer posting, confessing she has become an obsession that threatens his ability to function as a surgeon. Lilly refuses, insisting she has the same right to serve as anyone. Robbie declares they are "done" and from that point refuses to speak to her.

Lilly's aristocratic identity is exposed when a former footman recognizes her and calls her "Lady Elizabeth" in front of the camp. She confesses her privileged background to her friends, who accept the news, though several nurses begin taunting her. Robbie furiously defends Lilly but still does not speak to her directly. In December 1917, Lilly visits Edward in Saint-Omer, where he confides devastating truths: His engagement was arranged by their parents to clear his debts, he has grown emotionally numb to the war's horrors, and he has stopped believing he has a future. He rewrites his will in Lilly's favor and asks her to promise she will survive, travel, go to school, and find someone to love.

In March 1918, Lilly learns from the family butler that Edward has been reported missing in action after a raid. She travels to Paris, where Robbie is staying at the Ritz in the suite Edward had reserved for a trip the two men had planned. Over dinner at a tiny brasserie, they begin to rebuild what was broken between them. Robbie admits he was wrong, confessing that his desire for her nearly drove him mad. Lilly tells him she wants to look forward, not backward. Back at the hotel, they make love for the first time. Afterward, each confesses love for the other, and they agree that when the war ends, they will be together. For the first time in his life, Robbie experiences joy.

The German Spring Offensive begins shortly after, and the 51st is overwhelmed with casualties. For a month, Lilly drives endlessly while Robbie operates for twenty or more hours at a stretch. On the road to Saint-Venant one night, a shell explodes in Lilly's path. Her ambulance overturns and she is pinned beneath it with a compound fracture to her left leg and a lacerated spleen. Constance finds her. Lilly briefly regains consciousness and asks Constance to tell Robbie to forgive her. All five wounded men in the ambulance survive.

After the remaining staff and patients at the 51st are rescued, Robbie learns of Lilly's crash. He gives Constance a sapphire engagement ring he bought in Paris the day after Lilly left. Lilly wakes in the post-operative ward with Robbie at her side. He proposes marriage, promising that after the war he will resume his surgical career in London and she can attend university. Lilly accepts, and their gathered friends erupt in applause.

In the epilogue, set in January 1919, Lilly works as a clerk for the Imperial War Graves Commission, answering letters from families seeking the graves of loved ones. She lives in Camden Town and pays weekly visits to her parents, who remain broken by the apparent loss of Edward. Robbie has been kept in France after the Armistice, staffing hospitals overwhelmed by the influenza pandemic's deadly second wave.

One evening, Lilly receives a telegram and races to Victoria Station. As the crowd clears, she sees Robbie emerge from the last carriage, accompanied by Edward, alive but missing a leg and walking on crutches. Robbie spent months after his discharge searching for Edward, finally finding him. Edward tells Lilly that Robbie found him when he was "lost, even to himself." Lilly embraces her brother, then throws herself into Robbie's arms. He teases her about kissing in public while in uniform. She reminds him the war is over, and they kiss.