"The past is a foreign country: they do things differently there."
The greater portion of the text concerns itself with Leo's past, particularly the summer of 1900, spent in Norfolk as a guest at Brandham Hall, the luxurious country home of his schoolfriend Marcus Maudsley. Here the young Leo, on holiday from boarding school, is a poor boy among the wealthy, upper class. Leo's comparatively humble background is obvious to all and he does not really fit in there; however, his hosts do their best to make him feel welcome, treating him with kindness and indulgence. When Marcus falls ill, Leo is left largely to his own devices. He becomes a secret "go-between" for Marian Maudsley, the daughter of the host family, and nearby tenant farmer, Ted Burgess. At first, Leo is happy to help Marian because she is kind to him and he has a crush on her. Besides, Leo is initially ignorant of the significance or content of the messages that he is asked to carry between Ted and Marian. Leo is a well-meaning and innocent boy, so it is easy for the lovers to manipulate him.
The fact that Ted comes from a much lower social class than Marian means there can be no possible future in the relationship because of the social taboos involved. Although Marian and Ted are fully aware of this, Leo is too naive to understand why the lovers can never marry. The situation is further complicated by the fact that Marian is about to become engaged to Viscount Hugh Trimingham, a wealthy and influential member of the nobility who is also spending the summer with the Maudsley's at Brandham Hall. Together, these factors make Marian's secret relationship with Ted highly dangerous for all parties concerned.
Later, Leo acts as an interceptor, and occasional editor, of the messages. Eventually, he begins to comprehend the sexual nature of the relationship between Marian and Ted, and feels 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 is forced to continue. Ultimately, Leo's involvement as messenger between the lovers has disastrous consequences. The trauma which results when Marian's family discover what is going on leads directly to Ted's shotgun suicide. The experience profoundly affects Leo, leaving him with permanent psychological scars. He subsequently grows up to be an emotionally detached adult who is never able to establish intimate relationships. Although Leo eventually learns to live with what happened back in 1900, he never quite manages to move beyond it and live an emotionally fulfilled life. When he looks back on the events through the eyes of a mature adult, he is fully aware of how the incident has left its mark on him. In a final twist to the story, 52 years later, a very elderly Marian Maudsley persuades Leo (now aged in his 60s) to act as go-between one more time.
It may be said that Hartley was a brilliant thematic writer, in that many deliberate themes and metaphors may be found in his work. Within this novel, delicately nuanced themes such as the innocence of childhood and its loss, family life (or a lack of such), class and gender distinctions, and education are to be found. Hartley makes great use of the symbols such as the weather, Atropa belladonna, Zodiac signs and the colour green to highlight the main themes.
The plot has some parallels to that of a classical Danish novel, Brudstykker af en Landsbydegns Dagbog ("Fragments from a Parish-Clerk's Diary") by Steen Steensen Blicher (1824). Michael Frayn's novel, Spies, is said to draw heavily from this book. Ian McEwan's novel Atonement examines some similar themes and has some loose plot similarities.