In the book's prologue, Leo Colston chances upon a diary from 1900, the year of his thirteenth birthday, and gradually pieces together a memory that he has suppressed. Under its influence, and from the viewpoint of what he has become by the midpoint of "this hideous century", Leo relives the events of what had once seemed to him its hopeful beginning. The importance of his boarding school's social rules is another theme running through the book and complicates Leo's interaction with the adult world.
"Curses" of his devising had routed boys who were bullying Leo at school and had given him the reputation of a magician, something that he came to half-believe himself. As a result, he is invited as a guest to spend the summer at Brandham Hall, the country home of his school friend Marcus Maudsley, who comes from a wealthy landed gentry family. There the socially clumsy Leo, with his regional accent, is a middle-class boy among the wealthy upper class. Although he does not fit into this society, his hosts do their best to make him feel welcome, treating him with kindness and indulgence, especially their daughter Marian.
A footpath through marshy ground on the way to Bradenham, the Norfolk location of the novelWhen Marcus falls ill, Leo is left largely to his own devices and becomes a secret "postman" for Marian and nearby tenant farmer Ted Burgess, with whom she is having a clandestine relationship. Leo is happy to help Marian because he has a crush on her and likes Ted. Besides, Leo is initially ignorant of the significance or content of the messages that he is asked to carry between them, and the well-meaning, innocent boy is easily manipulated by the lovers. Although Marian and Ted are fully aware of the social taboo that must make their relationship a matter of the utmost secrecy, Leo is too naïve to understand why they never can marry. The situation is complicated by the fact that Marian is about to become engaged to Hugh, Viscount Trimingham, the descendant of the area's nobility who formerly lived in Brandham Hall.
As he begins to comprehend that the relationship between Marian and Ted is not to do with "business" as they have claimed, Leo naively believes that Marian's engagement ought to bring the correspondence between her and Ted to an end. Feeling increasingly uncomfortable about the general atmosphere of deception and risk, Leo tries to end his role as go-between but comes under great psychological pressure, and he is forced to continue. Ultimately, his unwilling involvement has disastrous consequences when Marian's mother makes him accompany her as she tracks the lovers to their hiding place and discovers them having sex. The trauma that results leads directly to Ted's suicide and Leo's nervous collapse.
In the epilogue, the older Leo summarises how profoundly the experience has affected him. Forbidding himself to think about the scandal, he had shut down his emotions and imaginative nature, leaving room only for facts. As a result, he never has been able to establish intimate relationships. Now, looking back on the events through the eyes of a mature adult, he feels it is important to return to Brandham some 50 years later in order to tie up loose ends. There he meets Marian's grandson and finds Marian living in her former nanny's cottage. He also learns that Trimingham had married Marian and acknowledged Ted's son by her as his own. Trimingham had died in 1910; Marcus and his elder brother had died in World War I, and Marian's son died in World War II. In the end, the elderly Marian persuades Leo, the only other survivor from her past, to act once more as go-between and assure her estranged grandson that there was nothing to be ashamed of in her affair with Ted Burgess.
