The elderly protagonist, Stephen Wheatley, narrates the story of his childhood, after fifty years have passed. The unique scent of privet in the summer air evokes childhood memories and encourages him to travel from Germany, where he lives, back to the Close, the English neighborhood where he grew up. When he arrives, the familiar yet different neighborhood brings back new, specific memories.
Stephen then recounts the story of his childhood, flashing back and forth throughout the novel from the present to the past, when he was an awkward young boy who followed around his best friend, Keith Hayward. He explains that Keith and his family were very well-off, and that he felt fortunate to be Keith’s friend. Stephen was the only person in the Close—other than Auntie Dee, Keith’s mother’s sister—who frequently visited the Haywards’ house.
Stephen begins his reminiscence on the fateful day when Keith says six simple words that “turned our world upside down.” Although he struggles to establish the exact order of the events in which they happened, Stephen states that Keith’s six words were: “My mother is a German spy.”
From that moment onward, Stephen does not question Keith’s claim, and the two begin devising an undercover mission to spy on Keith’s mother. They convert a used notebook into a logbook and carry out their first investigation in Keith’s mother’s sitting room. Stephen and Keith find her diary and make note of little “x” marks in her calendar that occur once a month. They attribute it to a notation for secret meetings, but it is more likely that the x’s simply mark her menstrual cycles. Stephen, from the present, notes that this is another turning point in the story.
Stephen fast-forwards the narrative to when he and Keith create an official hiding spot where they can spy on Keith’s mother in the privet hedges that adorn the front of Miss Durrant’s bombed house. They swear to never tell anyone about their secret mission, and Keith erects a sign labelled “Privet” (“private” misspelled) at the entrance of their concealed hangout.
Several of their initial observations of Keith’s mother prove to be unfruitful, but on one Saturday, Stephen and Keith see Keith’s mother going to Auntie Dee’s and receiving a grocery list to do Auntie’s dee shopping. Keith runs after her, with Stephen immediately behind, but they lose her at the turn of the street. As they start conjecturing where she could have disappeared to, they find her back at Auntie Dee’s house again. The same situation, where Keith’s mother disappears at the corner as the boys follow her, happens a few more times. In one instance, a girl in the neighborhood, Barbara Berrill, bothers them in their hiding spot and tries to expose their undercover operation. However, just as their efforts begin to seem futile, Stephen notices Keith’s mother rubbing slime off her hands as she reprimands Keith for staying out late.
Stephen pauses the story and returns to the present, where he describes the geographical arrangement of his childhood town. He explains that the Close had been newly-built when he first moved there, as an outshoot from the railway. At the end of the Close, the street became the Avenue to the left, whereas the right led to a tunnel that connected with the Lanes, a narrow trail that petered out into empty green fields. Beyond the Lanes were the Cottages, a dismal village of dirty children and vicious dogs, an abandoned farm, and finally, a no-man’s land where development ceased at the start of the war (called “the Barns”).
Stephen explains that the source of the slime on Keith’s mother’s hands was the tunnel. Thus, he pieces together the mystery of her disappearance: she turns right at the end of the street and goes into the tunnel. Stephen and Keith go to the tunnel to explore, during which they pass by Keith’s mother, who is holding a letter. Beneath the undergrowth beyond the tunnel, they find the box of an old croquet set and a package of cigarettes labelled with an “X” inside.
The next day, Stephen is waiting in the hideout alone when Barbara joins him. Barbara makes several suggestions about why Keith’s mother is always doing Auntie Dee’s shopping. She speculates that Keith’s mother might be buying items from the black market, or taking a message to Auntie Dee’s secret boyfriend. Meanwhile, Stephen ignores her gossip and feels guilty about letting her invade his and Keith’s secret space.
The following day, Stephen is alone again in the privet hedges when Keith’s mother comes into the hiding place. She questions Stephen about his and Keith’s game of spying and gently warns him that it could potentially be insulting to the neighbors. She informs Stephen that she may not allow Keith to play with him if they don’t stop spying, and asks Stephen to keep their conversation a secret. That night, Stephen ventures out to the box by the tunnel to definitively prove that Keith’s mother is a spy, but he finds that a mysterious man, whose face he does not see, is there. The man runs away and the terrified Stephen returns to the Close to his worried parents. Under the streetlights, he discovers that he brought back a sock from the croquet box.
Stephen and Keith go back to the tunnel together and find that the box has disappeared. They then hear footsteps that go up into the Lanes and decide to follow the sounds. They go all the way past the Cottages to the Barns, where they discover someone hiding in an underground hideout, concealed by an old corrugated iron sheet. The boys start hitting the iron sheet with sticks until they become frightened that the man behind it has died from fear. When they run back to the Close, they find Keith’s father impatiently waiting for Keith’s mother, who has not returned from her errands. Keith’s mother immediately appears and asks Stephen, “Was it you two?”
The narrative returns to the present, with Stephen questioning what his younger self had known at that point. He states that he felt guilty for breaking his promise to Keith’s mother and that he had probably both believed and not believed that she was a German spy. He recounts that he eventually built up the courage to knock on Keith’s door, after not having seen him since the last time. He finds that Keith has abandoned their undercover mission and he accepts that everything is back to normal. Keith’s father asks Keith for a missing thermos, but Keith does not know its whereabouts and is beaten as a result. At that point, Stephen becomes aware of the fact that Keith’s mother has taken the thermos to the Barns. He runs into her in the tunnel, getting slime on her dress, and tells her that Keith will be punished again if she does not bring the thermos back home.
After this incident, Stephen no longer plays with Keith, and he realizes that he has been pushed out of the Haywards’ world forever. He still observes the Haywards from the lookout and rarely sees Keith’s mother leave the house, which Barbara believes may be because of Keith’s mother’s secret boyfriend. Meanwhile, Barbara states that her older sister, Deirdre, and Stephen’s brother Geoff have been smoking and kissing in the hideout.
One afternoon, Keith’s mother pays Stephen a visit in the lookout to ask him to deliver a shopping basket to the mysterious man, who she says is very sick. After Stephen agrees and Keith’s mother leaves the hedges, Barbara immediately comes in to ask what Keith’s mother had wanted. She takes out a cigarette that they smoke together, they look through the contents of the basket, and, eventually, they kiss. Stephen and Barbara find a letter in the basket, which Barbara tries to open. Suddenly, Keith’s father appears and asks to have a word with Stephen—and tells him to bring the basket.
Stephen follows Keith’s father to his garage and Keith’s father tells Stephen to stop playing silly games. Though Stephen initially resists, he leaves the basket with Keith’s father and returns home crying, unable to tell his parents what has upset him. The next day, Stephen returns home from school and assembles a makeshift package of food and medicine in a satchel to deliver to the mysterious man. At the Barns, he leaves the contents of the satchel near the iron sheet. The man calls out Stephen’s name.
Again the story returns to the present, and Stephen wonders if he had understood then who it was that had called out his name. He describes the voice as familiar, not at all foreign or tramp-like. He says that the voice asks for Keith’s mother, but Stephen tells him that she can’t come. The man then asks how Auntie Dee and her daughter Milly are doing. When Stephen tries to leave, the man tells him that he had always loved Keith’s mother and asks Stephen to give her a patch of silk with a map of Germany drawn on it and to tell her “forever.”
Stephen waits in the lookout and concludes that Keith will not come to their hiding place again. But after dinner, he finds Keith waiting for him, angry that Stephen let Barbara into their spot and showed her all their things. Keith tortures Stephen by pushing his knife against Stephen’s throat until he draws blood. Stephen realizes that Keith must have learned this technique from his father, which explains why Keith’s mother wears a scarf around her neck even in the summer. Stephen returns home and is unable to hide his wound from his parents. His father cleans the wound, and soon afterwards Stephen falls asleep. When he wakes up in the middle of the night, he decides to go to the Barns to hide the silk map. But he discovers other men at the Barns, collecting the mysterious man’s body, which had been hit by an incoming train.
Stephen begins bringing the story to a close, as (in the present) he revisits the tunnel and the Lanes and thinks about the hidden scarf. He describes what had happened after that night: he never played with Keith again, Uncle Peter had gone missing, and there was a falling out between Auntie Dee and Keith’s mother. Stephen finally reveals that he himself was the secret German, because he used to be “Stefan Weitzler.” He explains that his family had moved from Germany before the war had started and, since his mother was English, they all became the Wheatleys. He also reveals that they are Jewish, though Stephen hadn’t known this growing up.
After his parents died, Stephen returned to his homeland, because he felt that he had “never really taken to life” in England. In Germany, he had a rough start with learning an unfamiliar language in a new environment, but he became a professional translator and met a German woman, whom he married and started a family with. Stephen then says that his father was actually a spy—he was a German man working for the British war effort. He continues giving accounts of what happened to the others who lived in the Close, including Keith, who became a barrister. Stephen finally reveals that the mysterious man was Uncle Peter.