Poirot has gotten a letter challenging him to stop a murder. He is given the date and location, the letter is signed ABC. The police dismiss the incident as yet another harmless crank letter - until a murder takes place on the day and time stated in the letter and an ABC railway guide is found on the scene. More letters arrive and a pattern begins to form, the killer is working his way through victims and towns alphabetically - Ascher in Andover, Barnard in Bexhill, Clarke in Chruston... Poirot and the police are in pursuit but always it seems a frustrating step behind. Ultimately Poirot is successful of course. The solution to the crime is clever and original, even by Christie standards.
This is a departure from the usual 'cozy' style that is more typical of Christie (ie confined location, murderer and victim know each other, motive clearly established, little focus on the crime itself). This is darker than her usual work, the victims are seemingly chosen at random, the entire country is threatened, and the messages from the killer are reminiscent of Jack the Ripper.
Poirot gives a description of the killer based on the letters and evidence collected at the crime, in a manner that is very like a modern day profiler. Keep in mind that this book was written nearly 80 years ago.
If you are a Christie fan this is definitely a must read but if you are looking for a more comfortable 'cozy' you may find this one a bit disturbing.