Poirot returns to Britain after a long absence anxious for a new case which will challenge his gray cells. He quickly gets his wish in the form of taunting letters from a serial killer who has dubbed himself ABC and leaves a railroad schedule at the scene of each crime. The victims as well as the crime scenes appear to be chosen randomly, but maintain an obsessive adherence to alphabetical order. However, Poirot grows to believe that the killer is not the madman the authorities believe, but a methodical killer with a very tangible motive.