The first chapter introduces Stephen, the main character and narrator, an elderly man who recalls his past triggered by a familiar smell. While walking with his daughter and granddaughter, he encounters the same scent again. They identify it as coming from a common German bush called "linguster." Motivated by these memories, Stephen decides to revisit his childhood home in London.
Upon his return to England, Stephen visits his former house from 50 years ago. He realizes that although it initially appears unchanged, the environment has transformed drastically and no longer resembles his childhood abode. Despite questioning his reasons for returning, Stephen's conviction is reinforced when he hears the familiar sound of a train from his childhood. He reminisces about his neighbors, particularly the Pincher family living next door at No. 2, who were shunned due to their untidy lawn.
In a dreamlike sequence, Stephen sees his younger self exiting the house and roaming the neighborhood. He mentions Barbara and Deirdre, two girls known for their wild behavior due to their father's absence during the war. Stephen also notes that his older brother, Geoff, spent time with one of the girls. However, Stephen's closest friend was Keith, who lived at No. 9, came from a wealthy family, and preferred playing at his own lavish house.
Keith's parents, who did not work and eagerly awaited the end of the war, fascinated Stephen. Keith's father, a soldier in the First World War, often boasted about his role and bravery. Stephen also had a good rapport with Keith's aunt, Dee, whose husband was idolized and went off to fight. Stephen reflects on his own family, emphasizing their lack of distinction but the underlying love that binds them as relatives.
During a summer day, Keith informs Stephen that he suspects his mother is a German spy. Stephen is initially shocked, but soon compares his own parents unfavorably to Keith's. Stephen begins piecing together clues, speculating that Mrs. Hayward, their neighbor, sent numerous letters as a spy. He reflects on how the bombing of a nearby house might be connected to Mrs. Hayward's true identity. Looking back, Stephen regrets never questioning Keith's motives or the reliability of his information.
The two boys start surveilling Keith's mother during her daily errands, taking notes on her interactions and growing suspicious of those she associates with. In their exploration of her sitting room, they find a diary marked with an X for each month. Startled by her return, they hide and retreat to discuss in the shrubs near Durrant's house.
Stephen suggests various approaches to handle the situation, but Keith dismisses his ideas. Keith reveals an old knife and makes Stephen swear secrecy regarding his parents. Keith implies that he will confront his mother on the night marked with an X. Despite facing bullying at school, Stephen derives strength from the shared secret. At home, while his father attempts to engage him in conversation, Stephen remains distant and easily distracted.
Stephen manages to leave his house one night and heads to the hideout but is discovered by Keith's mother, who sends him back home. The boys spend an entire day spying on Keith's mother but witness nothing significant. Stephen proposes the possibility of his own father being a spy due to his extended absences and meetings with strangers speaking a foreign language.
As they prepare to return home, they spot Keith's mother leaving the house and heading towards Dee's residence. They follow her but quickly lose track. Over the following days, they attempt to locate the secret passageway used by Keith's mother, but their efforts prove futile.
Uncle Pete, Dee's husband, returns from the war, eliciting admiration for his uniform. While Stephen and Keith continue observing Keith's mother, they often lose her and occasionally encounter her accidentally.
In the fifth chapter, Stephen describes how the town has changed since his childhood, highlighting the impact of the war.
Returning to the narrative, Stephen recalls how he and Keith ventured through a tunnel used by Keith's mother, leading them to suspect that she might be building a bomb to target passing trains. While hiding, they discover a buried box containing cigarettes and a piece of paper marked with an X. They are startled when Keith's mother returns, prompting them to retreat and discuss their findings near Durrant's house.
Stephen continues spying on Keith's mother in Keith's absence. One day, Barbara joins him in the hideout, and they observe Keith's mother shopping in the evening, finding it peculiar. Barbara presents two plausible theories about Keith's mother: she might be involved in the black market, or she is aiding Dee in hiding an affair. Before leaving, Barbara mentions that many women have affairs while their husbands are at war, suggesting it shouldn't be surprising.
Stephen persists in spying on Keith's mother, even though Keith no longer plays with him. One day, Keith's mother approaches him, indicating that she knows about his activities. She warns Stephen that Keith will no longer be allowed to play with him if they continue spying. Before leaving, she insists that their conversation must remain a secret.
Stephen distances himself, feeling guilty for failing Keith. However, one night, he decides to investigate the mysterious box alone in the tunnel. In the tunnel, he encounters a man but avoids looking at him. When the man flees, Stephen returns home and faces his worried parents' scolding. He withholds information about his whereabouts and refuses to reveal an item he took from the box—a blue sock.
Stephen shares his discovery with Keith the next day, and they investigate the tunnel together. To their dismay, the box has vanished. Keith becomes angry, blaming Stephen for their failure. As they hide from Keith's mother, they witness her destination, but Stephen resists following her, and Keith is too fearful to do so alone.
The boys continue walking until they reach the Barns, an area where a man resided during the previous winter. They throw rocks at the spot where the man lived before heading home. Later, Keith's mother confronts them, inquiring if they were the ones throwing rocks at the Barns.
Following the incident, Keith no longer plays with Stephen, but he remains in the hideout, spying on Keith's mother and contemplating the evidence suggesting she is a spy. One day, Stephen visits Keith's house and notices the changes in Keith's father's behavior, witnessing him beating Keith and acting strangely when discussing his wife. Keith is beaten for losing a thermos, and Stephen knows it was Keith's mother who took it. Stephen finds Keith's mother in the tunnel and hurriedly informs her about Keith's mistreatment.
Subsequently, Stephen rarely interacts with Keith or Keith's mother. Whenever he sees her, she is accompanied by her husband. Barbara theorizes that perhaps Keith's mother was caught having an affair, which explains why her husband restricts her freedom.
One day, while Stephen is in the lookout, Keith's mother attempts to approach him, possibly to request his assistance in delivering a letter. However, she stops when she notices Stephen with Barbara.
Distressed by the realization that others have discovered his hideout, Stephen spends more time inside with Barbara. They develop another theory regarding the mysterious man, speculating that he is a wounded German soldier being cared for by Keith's mother.
The eighth chapter concludes with Stephen revealing that Keith's mother approaches him, asking for a favor. She entrusts Stephen with a basket of groceries and requests that he deliver it to the man. The man is ill and lacks food, so Keith's mother provides for him. Stephen accepts the basket and promises to fulfill her request.
Barbara joins Stephen in the hideout, and they are discovered by Stephen's father, who instructs Stephen to take the basket with him. Stephen's parents take the basket, leaving Stephen feeling guilty, worried that the man will go hungry.
The following day, Stephen gathers provisions from his parents' pantry and embarks on his mission to deliver the groceries. As he approaches the entrance of the hideout, he hears the man coughing from inside. Stephen leaves the basket near the entrance, preparing to depart, when a familiar voice calls his name. The voice is British and reminiscent of someone he knows. Curiosity piqued, Stephen inquires about Keith's family. Frozen in place, he leaves only when the man hands him a piece of silk, instructing him to give it to Keith's mother.
Stephen attempts to contact Keith's mother and spots her following her sister, but she is denied entry into her sister's house. Stephen then encounters Keith, who is enraged that someone else has been in their hideout and even injures Stephen with the knife they kept there.
Returning home, Stephen is cared for by his parents, though he refuses to answer their questions. Later that night, he attempts to return the scarf to the Barns but comes across an ambulance. It becomes evident that the man has taken his own life by jumping in front of a train. Stephen leaves the scarf behind and heads home.
In the final chapter, Stephen discloses the identity of the mysterious man. While not entirely certain, it is strongly suggested that the man was Dee's husband, Pete, who deserted from the army and was engaged in an affair with Keith's mother. Stephen and Keith's friendship disintegrates.
During that summer, Stephen discovers his family's German heritage. His mother is British, while his father moved to England after losing his entire family in the war. When Stephen grows older, he travels to Germany and starts a family there. It is revealed that his father was a German spy working for the British government.
The book concludes with Stephen standing in the streets, engulfed in the familiar scent that drew him back to his childhood home. The memories of his past, both thrilling and haunting, continue to resonate within him.
