James Zemboy observes that this novel lacks the unity of plot of a proper novel. It is a series of episodes, only unified by the theme of Hercule Poirot investigating and uncovering the identity of one of the villains. The Big Four themselves are unique characters, each one representing a personification of evil. But Zemboy finds these characters lacking in traits to make them amusing, engaging, or personally interesting. The minor characters are not unique. They are generic messengers or information providers. John Ingles serves only to provide information on Li Chang Yen, Flossie Monro is only significant in providing a single clue, and Sonia Daviloff only serves to show Poirot the position of the chess table.