Recovering from injuries sustained in a wartime flying accident, Jerry Burton and his sister Joanna leave London for the quiet country town of Lymstock, where the doctor has prescribed rest and fresh air. The pace suits them, and they settle in among neighbours who seem ordinary and outwardly content. Then an anonymous letter arrives, crude and venomous, accusing the siblings of being lovers rather than relations. They soon learn that such poison-pen letters have been spreading through the town for weeks, each one aimed at a different recipient and each one trafficking in private shame, real or invented. At first the campaign looks like the work of a frustrated, malicious crank, an unpleasant nuisance to be endured. Matters darken when one of the letters appears to drive a woman to take her own life, and the authorities are forced to treat the writer as something more dangerous than a busybody. As suspicion settles over respectable households, Jerry watches the town turn inward and wary, and finds himself drawn to the brittle, overlooked young Megan Hunter. The investigation stalls until a clergyman's wife sends for an elderly visitor with a long memory for human wickedness. Narrated by Jerry himself, the story studies how easily a community can be poisoned from within, and how a sharp observer reads the difference between what people show and what they hide.