Adapted by Mark Gatiss (“Sherlock”), “The Big Four” plunges Poirot into a world of global espionage, set against the backdrop of the impending WWII. The public is in a panic after the shocking death of a Russian chess grandmaster, and Poirot must navigate international figures and intrigue to identify the culprit, with the help of old friends Captain Hastings, Inspector Japp and Miss Lemon.