Set during World War I, the story follows Alfie Summerfield, a young boy in London, as he discovers the truth about his father's disappearance and attempts to bring him home.
On July 28, 1914, Alfie turns five, the same day war breaks out across Europe. His parents, Georgie and Margie Summerfield, throw him a party, but most neighbors cancel. Only a handful attend: Granny Summerfield; Old Bill Hemperton, an elderly neighbor; and Alfie's best friend, Kalena Janáček, with her father, Mr. Janáček, a Jewish shopkeeper from Prague. Georgie, a milkman, comes home looking worried. That evening, Margie begs him not to enlist, but Georgie reassures her the war will be over by Christmas. The next morning, he walks in wearing a soldier's uniform. He has enlisted. Margie runs upstairs in tears.
Alfie's memories trace the close-knit community on Damley Road and the changes the war brings. Joe Patience, Georgie's oldest friend, is independent-minded and refuses to enlist. As wartime hostility intensifies, the Janáčeks become targets despite being from Prague, not Germany. Their shop windows are smashed and "No Spies Here!" is painted on the door (31). Classified as persons of special interest from the Austro-Hungarian Empire, a wartime category applied to civilians from enemy nations, the family is forcibly taken to an internment camp on the Isle of Man. When Margie can barely afford rent and food, Alfie breaks into the Janáčeks' empty house and takes Mr. Janáček's shoeshine box, intending to use it to earn money.
Four years into the war, Alfie is nine. Margie works as a Queen's Nurse, takes in laundry, and does sewing. She is exhausted and rarely laughs. Each morning, Alfie scans the newspaper's casualty lists for his father's serial number, 14278. He attends school only two days a week, spending the rest as a shoeshine boy at King's Cross Station. He secretly slips most of his earnings into Margie's purse.
Margie has told Alfie that Georgie is on a secret mission for the government. Alfie does not believe her. Early letters from Georgie are cheerful, but after he reaches France, the tone changes drastically. Though Margie stops sharing the letters, Alfie finds them hidden under her mattress. The final letters are fragmented and incoherent, and eventually they stop altogether. On Alfie's ninth birthday, he confronts Margie, demanding the truth. She slaps him, the first time either parent has ever struck him, then breaks down sobbing. Alfie remains convinced his father is dead.
One afternoon at King's Cross, a military doctor sits down for a shoeshine. When wind scatters his documents, Alfie helps gather them and spots an official page from the East Suffolk & Ipswich Hospital listing patient names and serial numbers. One entry lists his father's name and serial number. Alfie is stunned. His father is alive, in a hospital in England.
Alfie travels to Ipswich. On the train, he meets Marian Bancroft, a young woman who works with traumatized soldiers. She introduces him to the term "shell shock," a condition affecting soldiers whose minds have been damaged by war. At the hospital, Alfie encounters disturbing scenes: patients who react with terror to ordinary sounds and men with catastrophic injuries. In a secluded garden, he finds his father sitting in a wheelchair. Georgie is gaunt and does not recognize him. Alfie retrieves a newspaper, folds it in half, and pulls from his pocket an apple drop Marian gave him. He says, "Look what I've got for you" (144), mimicking the Saturday ritual in which Georgie used to hide sweets in the newspaper for Alfie. Recognition slowly dawns, and Georgie whispers Alfie's name.
Back at King's Cross, Alfie shines the shoes of a man who turns out to be Prime Minister David Lloyd George. Margie appears at the station without noticing Alfie. She tells the prime minister her husband has shell shock and urges him to act on the condition. She boards a train to Ipswich, confirming for Alfie that his mother has been visiting Georgie all along. Later, Dr. Ridgewell, the military doctor whose papers revealed Georgie's location, returns for another shoeshine and explains his work treating soldiers whose minds have broken down.
Alfie visits Joe and asks for help breaking Georgie out of the hospital. Joe reveals he already knows about Georgie and has visited him weekly since his release from Wormwood Scrubs prison, where he served two years as a conscientious objector, a person who refuses military service on moral grounds. He describes the beatings he endured that left him with scars and a limp, and the humiliation of receiving white feathers, symbols of cowardice pressed on men not in uniform. Joe refuses to help, arguing the hospital is the best place for Georgie. In frustration, Alfie tears open a cushion, grabs a white feather, and presses it against Joe's chest. Joe utters Alfie's name with deep pain, and Alfie runs from the house.
Alfie returns to the hospital with civilian clothes and train tickets. He finds Georgie on the second floor. Georgie partially recognizes him but shifts between clarity and confusion. Alfie lies, telling Georgie that Dr. Ridgewell has approved his release. They dress Georgie in civilian clothes and slip out of the building. On the train home, a soldier taunts Georgie for not being in uniform, and at King's Cross the noise of engines and whistles makes him tremble.
At home, Georgie falls into the armchair and rambles incoherently, mixing memories of the trenches with fragments of his old life. Alfie takes Georgie to King's Cross to keep him occupied. When a train's doors slam in rapid succession, the sound triggers Georgie: He covers his ears and crouches as if preparing to go over the top of a trench. Alfie calms him, then runs to buy cigarettes Georgie has requested. While he is gone, another train's doors slam. When Alfie returns, Georgie has vanished.
Alfie rushes home in despair. Old Bill has told Margie he saw Georgie, and she, Granny Summerfield, and Old Bill confront Alfie. He confesses everything. Dr. Ridgewell arrives, having traced the missing patient to the Summerfield address. Then Joe Patience knocks and tells them Georgie is safe in his front room. Joe followed Alfie and Georgie to King's Cross and, when Georgie ran in panic, chased him and brought him home. He believes Georgie will recover if they all help. Granny Summerfield, who has shunned Joe for years, softens. She touches the scars on his face and links her arm through his as they walk to Joe's house.
Inside, Margie and Georgie hold each other on the couch. Granny Summerfield throws her arms around them. Georgie whispers, "Help me" (232). Alfie pushes through and locks his arms around his father's waist. A commotion erupts outside as neighbors pour into the street, crying and embracing. The war is over. Georgie closes his eyes, tears streaming, holding his son. There are still six weeks until Christmas, the deadline Georgie once promised the war would not outlast.
In an epilogue set in July 1922, Alfie turns 13. The Janáčeks have returned from internment but are leaving for Prague, unable to forgive their treatment. Margie has found her calling as a nurse. Georgie is back on the milk float with Alfie beside him; he still has nightmares but is working and healing. Joe has published a book, and Granny Summerfield has reconciled with him. Georgie thanks Alfie for finding him and asks why he went to such trouble. Alfie cannot say the words aloud, but his answer is clear: "For the best reason in the world. For love" (245).