This is the fifth story in the TARZAN adventure novels. Tarzan's enemy in this book is a Belgian traitor and an Arab chieftain named, Achmet Zek. Tarzan intends to 'liberate' more jewels from the lost city of Opar, unaware that his enemies know of his plan and they intend to liberate the jewels from Tarzan. The running battle becomes so intense that Tarzan is incensed to the point of losing his newly-found, refined demeanor and becomes the beast of old with a terrible ferocity. The apeman once again responds to the wild call and goes forth to prove that he is truly the king of the jungle...