It's December 19, 1931. Hercule Poirot and Inspector Edward Catchpool are looking forward to a much-needed, restful Christmas holiday, when they are called upon to investigate the murder of a man in a Norfolk hospital ward. Cynthia Catchpool, Edward's mother, insists that Poirot stay with her in a crumbling mansion by the coast, so that they can all be together for the festive period while he solves the case.

As Poirot digs into the mystery, he discovers that the murdered man was a retired post office master, and by all accounts very well-liked. The local constabulary's investigation failed to uncover how someone could have entered a hospital room and killed him under the noses of the staff. Cynthia's friend Arnold is soon to be admitted to that same hospital, and his wife is convinced he will be the killer's next victim, though she refuses to explain why.

With no obvious motive or suspect, Poirot has less than a week to solve the crime and prevent more murders, if he is to escape from this nightmare scenario and get home in time for Christmas. Meanwhile, someone else--someone utterly ruthless--also has ideas about what ought to happen to Hercule Poirot...