Birdsong opens in 1910 as Englishman Stephen Wrasyford arrives in the French city of Amiens to study textile manufacturing on behalf of his British employer. Stephen is tasked with observing the daily operations of factory owner René Azaire, and he boards in René’s home with René wife, Madame Azaire, and their two children, Lisette and Grégoire. Stephen soon learns that the local textile dyers have begun to strike on account of poor wages and working conditions, and while René does not employ any dyers, his own workers are disgruntled over the prospect of new technology eliminating their jobs. René fears a widespread workers’ strike, and he pays little attention to his life at home.
One day, Stephen finds Madame Azaire in the garden and learns that Lisette and Grégoire are not her children; the children’s real mother died just two years before, and she is René’s second wife. Stephen finds himself intrigued by René’s wife, and after he discovers her delivering packages of food to the striking dyers and their hungry families, Stephen falls in love with her. Meanwhile, Madame Azaire, whose name is Isabelle, is likewise intrigued by Stephen. Suffocating in a loveless marriage with a husband who beats her, Isabelle is open to his obvious advances, and they soon enter into a heated affair. Stephen and Isabelle meet frequently in the red room, a forgotten bedroom in the servants’ quarters of the Azaires’ home, and their love grows until Isabelle finally agrees to run away with him. They live a quiet life in a small French town until Isabelle becomes pregnant. Ashamed and confused, she leaves Stephen, never telling him about the baby, and returns to her family home and her beloved sister, Jeanne.
Stephen is heartbroken; however, he doesn’t look for Isabelle. He waits for a year in case she returns, and heads for Paris. He spends some time working as an assistant to a furniture maker, but his life is empty and meaningless. When World War I breaks out in 1914, Stephen is thankful for the distraction. He enlists in the army and his advanced education sees him through the ranks until he is soon a lieutenant fighting on the Western Front in France. The British trenches are filled with death and decay, and each day brings unspeakable horror, but Stephen finds comfort in his friendship with a fellow soldier named Michael Weir. Stephen is a world away from Isabelle and the red room, and he rarely thinks of her now. Each day is a struggle to survive, and as his men die all around him, Stephen is driven by his hate for his German enemy—and the men he commands. The war takes its toll on Stephen, and he nearly dies of an infection after he is injured in tunnel collapse; however, he is compelled to keep fighting. He believes that the war is nothing but “an exploration of how far men can be degraded,” and there is no end to the depravity.
Stephen’s story unfolds alongside that of his granddaughter, Elizabeth Benson, and her own life in London in the late 1970s. Elizabeth is single and fiercely independent. At thirty-eight, she is already the successful manager of a clothing design company, and she is also in love with a married man. Elizabeth is content in her life, despite its challenges, but she feels something is missing. One day, she reads a newspaper article about the anniversary of the 1918 armistice, and it touches a curiosity deep inside her. She knows her grandfather fought in the war, but little else about him. The topic seems too big and out of reach—it happened too long ago and in France—yet thoughts of it linger in her mind. She remembers seeing some of her grandfather’s old journals in her mother’s attic, and she decides to snoop a bit.
Elizabeth finds two journals, one of which is written in a strange code, and she gets very little information from them. Still, she senses that the war and her grandfather’s place in it has some importance in her life—even if she doesn’t know what that is. On her way to Brussels to visit her lover, Robert, Elizabeth stops at a World War I memorial in Albert, France. She finds a massive arch in an empty field engraved with British names. She learns that each name is a lost soldier—the unfound and presumed dead who had fallen on the surrounding fields. Elizabeth is overwhelmed by the size of the arch and the death it represents. While she has vague knowledge about a few key battles, she never truly appreciated the scale of the war. “Nobody told me,” she cries.
Back in 1917, the war rages on, and Stephen continues to fight. He finds himself back in Amiens after so much has changed. The city has previously been occupied by German soldiers and suffered subsequent bombardments, and Stephen barely recognizes the charred landscape. He soon discovers that Isabelle is still in town after a chance meeting with Jeanne, and he finds her disfigured and paralyzed. Isabelle further breaks his heart when she confesses her love for a Prussian soldier named Max. Stephen knows he must let her go, and he heads back to the front lines. The violence he finds there is unimaginable, and after an attack goes horribly wrong on account of bad planning, most of Stephen’s platoon is cut down by enemy fire.
Physically exhausted and mentally broken, Stephen is comforted in his new friendship with Jeanne, but he finds it difficult to go on after Weir is killed by an enemy sniper. Stephen is deeply ashamed of the war and what they have done, and in the final days of fighting, he is trapped deep underground with a dying miner named Jack Firebrace after a German bomb blows their tunnel. Stephen resigns himself to death, and when he is certain humanity can get no darker, he is saved by enemy soldiers. The war has ended, and as the guns silence and the birds sing, the men embrace each other.
In London in 1978, Elizabeth enlists her friend Bob to help decode her grandfather’s journals. She embarks on a journey to know her grandfather, and she begins with the men he served with. She locates his commanding officer alive and well in Scotland at eighty-eight years old, and he leads her to Brennan, a broken and forgotten soldier living in a veterans’ home for over sixty years. Elizabeth finds herself pregnant with her lover’s child, and while it seems that Robert will never leave his wife, she couldn’t be happier. She loves Robert, but more than that, the child satisfies a deep emptiness that she had not even been aware of. When Bob finally cracks her grandfather’s coded writing, Elizabeth finds unimaginable pain and suffering in the pages of the journals, but she also finds closure and hope. Through her journey to the past, Elizabeth learns the true story of her family and gains a new appreciation for the war and its cost to humanity. Elizabeth gives birth to a son she names John in honor of one of the men from her grandfather’s journals, and she is ready to embrace her future—with a firm understanding of her past.