It is December 19, 1931. Hercules Poirot and Inspector Edward Catchpool are called to investigate the murder of a man in the seemingly secure sanctuary of a hospital wing in Norfolk. Catchpool's persistent mother, Cynthia, asks Poirot to stay in a friends' mansion on the coast so that they can all be together for the holiday period while he solves the case. The owner of the mansion, Arnold Lorrier, is soon to be admitted to the same hospital, and his wife is convinced that he will be the next victim of the murderer, although she refuses to explain why. Poirot has less than a week to solve the crime and prevent further murders if he wants to escape from this nightmarish scenario and return home in time for Christmas. However, he himself ends up in the hospital, poisoned, and time is now counting down.