The novel picks up where Tarzan of the Apes left off. The ape man, feeling rootless in the wake of his noble sacrifice of his prospects of wedding Jane Porter, leaves America for Europe to visit his friend Paul d'Arnot. On the ship he becomes embroiled in the affairs of Countess Olga de Coude, her husband, Count Raoul de Coude, and two shady characters attempting to prey on them, Nikolas Rokoff and his henchman Alexis Paulvitch. Rokoff, it turns out, is also the countess's brother. Tarzan thwarts the villains' scheme, making them his deadly enemies. Later, in France, Rockoff tries time and again to eliminate the ape man, finally engineering a duel between him and the count by making it appear that he is the countess's lover. Tarzan deliberately refuses to defend himself in the duel, even offering the count his own weapon after the latter fails to kill him with his own, a grand gesture that convinces his antagonist of his innocence. In return, Count Raoul finds him a job as a special agent in Algeria for the ministry of war. A sequence of adventures among the local Arabs ensues, including another brush with Rokoff. Afterward Tarzan sails for Cape Town and strikes up a shipboard acquaintance with Hazel Strong, a friend of Jane's. But Rokoff and Paulovitch are also aboard, and manage to ambush him and throw him overboard.
Miraculously, Tarzan manages to swim to shore, and finds himself in the coastal jungle where he was brought up by the apes. He soon rescues and befriends a native warrior, Busuli of the Waziri, and is adopted into the Waziri tribe. After defeating a raid on their village by ivory raiders he becomes their chief. The Waziri know of a lost city deep in the jungle, from which they have obtained their golden ornaments. Tarzan has them take him there, but is captured by its inhabitants, a race of beast-like men, and condemned to be sacrificed to their sun god. To his surprise, the priestess to perform the sacrifice is a beautiful woman, who speaks the ape language he learned as a child. She tells him she is La, high priestess of the lost city of Opar. When the ceremony is fortuitously interrupted, she hides him and promises to lead him to freedom. But the ape man escapes on his own, locates the treasure chamber, and manages to rejoin the Waziri.