Tarzan and the Lost Empire, originally serialized in Blue Book magazine between October 1928 through February 1929, is the 12th book in the series. The story involves the search for Erich von Harben, a young German specialized in archaeology and dead languages, with a passion for mountain climbing. Von Harben was investigating the legend of the Lost Tribe of the Wiramwazi Mountains and disappeared. His father asks Tarzan for help, and Tarzan agrees.

In his search for Erich von Harben, Tarzan finds a lost remnant of the Roman Empire hidden in the mountains of Africa. They are inhabitants of two rival cities: Castra Sanguinarius, ruled by Sublatus Imperator, and Castrum Mare, ruled by Validus Augustus. Captured, Tarzan is forced into the arena as a gladiator...