The narrator, visiting Merchester for a tennis tournament, describes a household of friends gathered at the country doctor's secluded house; conviviality is shattered when a young woman is found murdered, apparently by poisoning. A formal investigation ensues, led by Detective Inspector Allport, with inquests, medical scrutiny and competing accounts from members of the party including Kenneth, the Tundish and Janet. The narrator examines clues, motives and alibis as tension rises through dining-room inquiries, midnight discoveries and a near-accident, leading to explanations, a challenge and the eventual unmasking of the crime.