The action is set during World War I. 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. (Wasimbu’s father Muviro, first mentioned in this story, goes on to play a prominent role in later Tarzan novels.)
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…