After a search for a legendary city of diamonds, Tarzan races with his pet lion Jad-bal-ja to save Hawks from being sacrificed to a lion-god.
Lord Greystoke, known as Tarzan to the natives of Africa among whom he lived, expects his wife and her niece with the young overseer of his estate. A renegade White meets the travellers and plans to attack them. But the timely arrival of the hero with his native army rescues them. A stranger in rags comes to the hero and informs him that he had escaped from the diamond city of a race of people little known. He tells the hero of a fabulous wealth in diamonds hidden within the Diamond palace of this tribe. The villain overhears him and during the hero's absence he abducts him and the young heroine. When the hero returns and hears of the abduction he takes his faithful lion along and rushes to their rescue, instructing his lieutenants to follow him wit his army of natives. Eventually he reaches the city of diamonds in time to rescue the heroine from being offered as a sacrifice to a lion, which the natives worshipped as their God.