Tarzan the Untamed 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.