Ah, finally, our first real classic. A great, inventive, rule-breaking classic. Before there was "Gone Girl," our inimitable Dame Agatha was playing the long narrative con. Plus, we get Poirot in all his masterful gamesmanship, sly as ever. A certain murderer may have wished that mustachioed Belgian had never retired and gone to grow vegetable marrows, but we (and justice seekers everywhere) are so much the better for his countryside foray.