Warning: Wikipedia contains spoilers.
In the ABC murders an apparent serial killer is murdering people in order of their names: first Annabel Ascher of Andover, second Betty Barnard of Bexhill, third Sir Carmichael Clarke of Churston, and so on. A man by the name of Alexander Bonaparte Cust is arrested for the murderers, but Poirot deduces that the murders were committed in order to hide one specific murder.