In 1933, retired detective Hercule Poirot is targeted by a taunting killer who sends letters signed "ABC", which Poirot must decode in order to discover the identity of the murderer.

Set in the 1930s, a time when Britain is dangerously divided and suspicion and hatred are on the rise, the story sees Poirot face a serial killer known only as A.B.C. As the body count rises, the only clue is a copy of The ABC Railway Guide at each crime scene. Poirot's investigations are continuously thwarted by an enemy determined to outsmart him. If Poirot is to match his nemesis, then everything about him will be called into question: his authority, his integrity, his past and his identity.