This whodunit series based on Agatha Christie's crime novels and short stories, is named after its star sleuth, Hercule Poirot, a famous former Belgian policeman, who settled for good in London after the war, soon so famous as an infallible private detective that he becomes a society figure in his own right. In each episode Poirot gets to solve a crime mystery -mostly murder(s)- for a paying client or otherwise catching his attention, generally along with his faithful English sidekick Captain Hastings and/or his Scotland yard 'friendly rival' Detective Chief Inspector Japp.

When a friendless old widow dies in the seaside town of Crythin, a young solicitor is sent by his firm to settle the estate. The lawyer finds the townspeople reluctant to talk about or go near the woman's dreary home and no one will explain or even acknowledge the menacing woman in black he keeps seeing. Ignoring the towns-people's cryptic warnings, he goes to the house where he discovers its horrible history and becomes ensnared in its even more horrible legacy.