S4 E1: A is for Andover, the town where Mrs Ascher is found battered to death. B is for Bexhill, where Betty Barnard is strangled. C is for Churston, the tiny Devonshire village where Sir Carmichael Clarke is murdered. Beside each body lies a copy of the ABC Railway Guide - open at the relevant page. The police are baffled. But the murderer has already made a grave mistake. He has challenged Hercule Poirot to unmask him.