Agatha Christie's The Mysterious Affair at Styles

Back from World War 1, Captain Arthur Hastings convalesces at Styles Court. But when the lady of the house suspiciously dies, the tranquil manor turns treacherous. The cause is murder and the killer could be anyone: her unappealing new husband, her ne’er-do-well son, the blunt female groundskeeper, even the local toxicologist. Enter Hercule Poirot who applies his “little grey cells” to solve the seemingly unsolvable mystery.