A routine dental appointment turns sinister when Hercule Poirot learns that the dentist who treated him that morning, Mr Morley, has been found shot dead in his surgery. The official verdict leans toward suicide, a conclusion that seems to firm up when one of Morley's other patients dies from an overdose of anaesthetic. Poirot refuses to accept the easy explanation. He retraces the handful of visitors who passed through the waiting room before the death: a powerful and discreet banker whose stability matters to the whole country, a Greek man with a shadowy reputation, and a talkative middle-aged woman whose account does not quite hold together. Working alongside Chief Inspector Japp, Poirot follows a trail that widens far beyond a single consulting room, touching on private fortunes, public anxieties, and the political currents running through Britain on the eve of war. Each stage of the inquiry is keyed to a line of the familiar counting rhyme, and the small inconsistencies Poirot collects gradually assemble into a pattern. As the connections between the patients sharpen, the case forces a question that reaches past any one crime: whether the safety of many can ever justify the cost paid by a few. Poirot must weigh order against justice while a calculating mind works hard to keep the truth buried beneath an ordinary tragedy.