A killer is working through the alphabet. In Andover, a shopkeeper named Ascher is found dead; in Bexhill, a waitress named Barnard; in Churston, a wealthy man named Clarke. Beside each body lies a copy of the ABC Railway Guide, opened to the page of the town where the victim died. Before every killing, Hercule Poirot receives a typed, mocking letter signed only with the initials A.B.C., naming the next place and daring him to intervene in time. With his old friend Captain Hastings narrating events, Poirot works alongside Chief Inspector Japp and the sceptical young Inspector Crome, but the police are left chasing a murderer who appears to choose strangers at random and strike across half of England. As the toll mounts, the relatives of the dead band together, determined to help the detective who cannot seem to get ahead of the next letter. Poirot grows convinced that the apparent madness conceals a precise design, and that the answer lies not in where the killer will go next, but in why these particular names were chosen. Christie tests her detective against a new kind of adversary, building a methodical puzzle around the unsettling idea of murder without obvious motive, and pressing Poirot to read the mind behind the alphabet before the list runs out.