Four and Twenty Blackbirds (S01E04)

Poirot is dining out at a London chop house one evening when eminent artist Henry Gasgcoine enters the restaurant for his customary dinner. The next morning, Poirot is intrigued to read in the newspaper that the painter has been found dead. When a post-mortem reveals that Gasgcoine died after eating only a little food, Poirot concludes that the man he saw dining in the restaurant could not have been Gasgcoine at all. (iTunes)