"Tarzan the Invincible," originally serialized in the magazine "Blue Book" from October 1930 through April 1931 as Tarzan, "Guard of the Jungle," is the 14th volume. In it, Tarzan, his monkey friend Nkima, and Chief Muviro and his faithful Waziri warriors prevent Soviet communists from looting the lost city of Opar. The story also prominently features Tarzan's lion ally Jad-bal-ja. This book marks the last appearance of Opar and La in the Tarzan series, aside from the 1936 juvenile piece "Tarzan and the Tarzan Twins with Jad-Bal-Ja the Golden Lion" (published later but chronologically earlier).