The Murder of Roger Ackroyd (1926)

Belgian Inspector Hercule Poirot has retired to the countryside in the small English village of King's Abbot. Dr. Sheppard, observing his new neighbor, is sure that he must be a former hairdresser. But the brutal murder of a local squire reveals the truth: the peculiar little man is actually a detective par excellence. The Murder of the wealthy industrialist Roger Ackroyd begins the night before with the suicide of Mrs. Ferrars, a wealthy widow. Her death is believed to be an accident, until Roger Ackroyd is stabbed to death in his locked study. There are rumors she poisoned her first husband, rumors that she was being blackmailed, rumors that her secret lover was Roger Ackroyd, a man who knew too much, but no one is sure.

There's no shortage of suspects, all the members of the household stand to gain from his death, from Roger's neurotic sister-in-law who has accumulated personal debts, to a parlormaid with an uncertain history who resigned her post the afternoon of the murder. But the police focus on Ralph Paton, Ackroyd's stepson and heir, and the person with the most to gain from Roger's death. When sleuth Hercule Poirot, who is living quietly in King's Abbot, agrees to investigate, the case takes a completely different turn. Poirot exonerates all of the original suspects, and lays out a completely reasoned case that the clever and devious murderer is someone who had not come under suspicion at all - someone whose motive has nothing to do with money.