Black adds that Christie's work is, in its way, typical of the literature of the interwar period, much of which reflected a concern about foreign threats and links between domestic and international challenges. The Big Four, the characters, are positioned as the hidden cause and connecting threat between the world-wide unrest, labour disputes, and the revolutions of the period—in particular, the October Revolution, with Vladimir Lenin and Leon Trotsky described as their puppets. The Big Four also have advanced technology in their arsenal.