Retired spy George Smiley is contacted by a wartime colleague, Ailsa Brimley, who now edits a small Christian magazine. She tells Smiley that she has received a letter from a reader, Stella Rode, claiming that her husband is plotting to kill her. The woman's husband is a teacher at a public school in the town of Carne. It so happens that Terence Fielding, brother of a classics professor who was one of Smiley's close associates in British intelligence during the war, is a house master at the school. However, before Smiley can intercede, Stella Rode is murdered. Feeling obliged to Brimley and guilty over the woman's death, Smiley travels to Carne to investigate.

Smiley's estranged wife, Ann, lived in Carne as a child, and upon his arrival, he becomes the subject of snide gossip. He also is witness to an invidious class division between 'town and gown' which is superimposed upon a religious division between adherents of the Church of England and Nonconformists. As the wife of a public school teacher, and as a nonconformist, Stella Rode occupied a low rank in the local social hierarchy, especially in the estimation of Carne's upper crust.

The town police focus on a homeless madwoman as the murderer, but both Smiley and the investigating officer believe her to be innocent. Brimley discovers the murderer's hidden blood-stained clothes, while in the meantime a boy in Fielding's house becomes the second murder victim. Stanley Rode admits to Smiley that, behind her apparent piety and ostentatious good works, his murdered wife was a pathological liar and schemer who would emotionally abuse him and viciously beat her own dog. Digging deeper, Smiley learns that Stella habitually humiliated, blackmailed and otherwise terrorised those around her, using both her mask of civility and a fear of reprisals to escape suspicion or retribution for her behaviour.

Smiley follows the clues to identify the real murderer, Terence Fielding, whom Stella Rode had been blackmailing over a wartime homosexuality conviction. The conviction was known to school authorities, who took advantage of the situation to themselves blackmail Fielding into remaining in his position at a significantly lower salary than his peers. The murdered boy had inadvertently made a discovery that would derail Fielding's attempt to implicate Stanley Rode and that would draw suspicion on Fielding himself instead. Incidentally, the boy was never aware of the significance of what he had seen before falling victim himself.

Fielding ultimately fails to frame Stanley Rode for the murders. Having admitted his guilt, Fielding is arrested.