Edgar Rice Burroughs

Tarzan returns to Opar, the source of the goldwhere a lost colony of fabled Atlantis is located, in order to make good on some financial reverses he has recently suffered. While Atlantis itself sank beneath the waves thousands of years ago, the workers of Opar continued to mine all of the gold, which means there is a rather huge stockpile but which is now lost to the memory of the Oparians and only Tarzan knows its secret location. A greedy, outlawed Belgian army officer, Albert Werper, in the employ of a criminal Arab, secretly follows Tarzan to Opar. There, John Clayton loses his memory after being struck on the head by a falling rock in the treasure room during an earthquake. On encountering La, the high priestess who is the servant of the Flaming God of Opar, and who is also very beautiful, Tarzan once again rejects her love which enrages her and she tries to have Tarzan killed; she had fallen in love with the apeman during their first encounter and La and her high priests are not going to allow Tarzan to escape their sacrificial knives this time. In the meanwhile, Jane has been kidnapped by the Arab and wonders what is keeping her husband from once again coming to her rescue. A now amnesiac Tarzan and the Werper escape from Opar, bearing away the sacrificial knife of Opar which La and some retainers set out to recover. There is intrigue and counter intrigue the rest of the way.

While John Clayton, Lord Greystoke (Tarzan) is away from his plantation home in British East Africa, it is destroyed by invading German troops from Tanganyika. On his return he discovers among many burned bodies one that appears to be the corpse of his wife, Jane Porter Clayton. Another fatality is the Waziri warrior Wasimbu, left crucified by the Germans. Maddened, the ape-man seeks revenge not only on the perpetrators of the tragedy but all Germans, and sets out for the battle front of the war in east Africa. On the way he has a run-in with a lion (or Numa, as it is called by the apes among whom Tarzan was raised), which he traps in a gulch by blocking the entrance. At the front he infiltrates the German headquarters and seizes Major Schneider, the officer he believes led the raid on his estate. Returning to the gulch, he throws his captive to the lion. Tarzan goes on to help the British in the battle in various ways, including setting the lion loose in the enemy trenches, and kills von Goss, another German officer involved in the attack on the Greystoke estate