They were critical of Syldavia's inclusion as an antagonist in the story, stating that the Syldavian attempts to kidnap Calculus "strains believability" because they had appeared as allies of Calculus and Tintin in both the preceding two-volume story arc, "Destination Moon" (1953) and "Explorers on the Moon" (1954), and in the earlier "King Ottokar's Sceptre" (1939).