Poirot is traveling on a flight from Paris to London aboard a small passenger plane. During the flight, a fellow passenger, Marie Morisot, a French moneylender, is found dead in her seat, seemingly having been poisoned. The poison is later identified as strychnine, and the situation becomes even more puzzling when it’s discovered that the victim had been sitting directly under the nose of Hercule Poirot, who didn’t see or hear anything unusual.