Returning from South America, Arthur Hastings meets with his old friend, Hercule Poirot, at his new flat in London. Poirot shows him a mysterious letter he has received, signed "A.B.C.", that details a crime that is to be committed very soon, which he suspects will be a murder. Two more letters of the same nature soon arrive to his flat, each prior to a murder being carried out by A.B.C., and committed in alphabetical order: Alice Ascher, killed in her tobacco shop in Andover; Elizabeth "Betty" Barnard, a flirty waitress killed on the beach at Bexhill; and Sir Carmichael Clarke, a wealthy man killed at his home in Churston. In each murder, an "ABC Rail Guide" is left beside the victim.