Tarzan drinks at a river when he smells an unfamiliar scent. He tracks it to a young woman captured by three unfriendly gorillas, the Bogani. They refuse his demands that they free her, so he ties up their leader with vines. The others back off.
The girl is Thea, from Athne, the city of ivory, days away. She recounts that she was riding her elephant when she was surprised by soldiers from an enemy city. She escaped, but was later captured by the gorillas. Tarzan offers to escort her back to her home since he's curious about this city he's never seen.
Nearing Athne, Tarzan sees Zandor, the city of gold, from which the soldiers who chased Thea came. She warns him that it's an evil city and that they should wait for dark to avoid patrols, but Tarzan wants to push on to avoid a coming storm. In the powerful storm, they have to wade through a swollen river. Tarzan submerges them to avoid an oncoming log in the river. On the bank, she's temporarily blinded by the water in her eyes. Tarzan blocks a tree felled by a lightning bolt, but it knocks the soil out from under his feet and he falls unconscious onto a floating log. He comes to before he goes over a waterfall, but can't pull himself out of the strong current. He calls the friendly Mangani apes, who pull him to safety. But he's immediately captured by Zandorian soldiers.
Tarzan is thrown into a cell where Phobe (pronounced fo-bay), the strongest warrior in Zandor. Phobe doesn't trust Tarzan or any outsiders. When they're summoned to see Queen Nemone, Tarzan's refusal to kneel insults her Tomos, her advisor, and interests her. Still, she orders Tarzan and Phobe to fight the next day in the arena. Tarzan easily fends off Phobe, infuriating him especially when Tomos eggs him on. He manages to grab Tarzan in a bear hug, which Tarzan breaks by hooking his legs over banner pole and lifting them both over, tossing Phobe onto the ground. Nemone orders him to finish Phobe, but Tarzan refuses. She frees Phobe, but orders Tarzan back to his cell. That night, she wants him to rule by her side, but he refuses.
One morning, Tarzan is summoned to the arena, where he is to watch the games as a captured, hooded and unarmed Athnean is put in with a black panther. Tarzan is outraged at the "sport" and jumps into the arena, defeating the panther. This infuriates Nemone and Tomos further, especially when the hood is removed and Tarzan and Thea embrace.
They're returned to prison. But Phobe sneaks into the prison, posing as a cleaner, and frees them. He sneaks them out and gives them a lion-drawn chariot, as well as his uniform. Tarzan and Thea almost make it out but a sentry on a catwalk sees Thea hunkered down in the chariot and they give chase. They almost catch Tarzan before he and Thea leap onto the lions and unhitch the chariot, which crashes into the following chariots. In the forest, they set the lions free and continue the trip to Athne.