Published in 1939, a few months before the cosy world of the English village was finally shattered, Christie here introduces us to Luke Fitzwilliam, an experienced police officer returning to England after a career in the Far East. After a string of several Poirot novels you sense she was wanting something fresh, someone fresh to write about.
Fitzwilliam has time and money on his hands - he's clearly in no hurry to find somewhere to live or an income to live on. Barely touched down on Blighty's shores, he makes the acquaintance of an old lady, the archetypal strangers on a train. She confides in him that she has uncovered a series of murders in her picturesque little village. He initially dismisses her as a confused old bird, but later has reason to believe she was on to something. He takes himself to the village and quickly establishes that there have, indeed, been a number of sudden deaths. It's a village which boasts two pubs, but only a handful of suspects (retired army officer, doctor, lawyer, antique dealer, and some other possibilities from the landed gentry and professional classes - well, you couldn't expect our hero to have to deal with the lower orders).
Fitzwilliam is not a Poirot. As a detective he is a bumbling fool. He is ludicrously old-fashioned and emotionally shallow in his relationship with women - he falls in love at the drop of a lock of hair, like a virginal adolescent, then seeks to win fair lady through bluster and the romantic gravitational pull of a detective launching into a compelling mystery ... enlisting the aid of a partner.
Red herrings abound, but in rather contrived fashion. Any reader looking to follow the detective skills of the hero will be left sitting in the sidings while the express train hurtles past. The story is plot driven - it relies a little heavily on coincidence: the characters are barely sketched (perhaps the most convincingly drawn character is a Persian cat). Superintendent Battle (who'd first appeared in "The Secret of Chimneys") makes a brief appearance, but the apprehension of the murderer requires luck rather than grey cells. You are left able to guess at the murderer, but rather through intuition and reading the writer's mind rather than following clues or unravelling a puzzle.
Entertaining, well-paced, but hardly a classic. You long for the return of the little Belgian.