It's 1933 and a killer travels the length and breadth of Britain via the railway network. The killer uses the alias ABC, and strikes in a methodical pattern, leaving a copy of the ABC railway guide at the scene of each of murder. As Poirot attempts to investigate he is thwarted on every front. If he is to match his most cunning nemesis, everything about him will be called into question: his authority, his integrity, his identity.

As A.B.C. dominates the news, the public are demanding answers, fearing for their lives. Without any warning, the A.B.C. murderer strikes again in the bathroom of Embsay station, where Ernie Edwards is murdered.