The story picks up with the Clayton family, Tarzan, Jane Porter and their son Korak, returning from their adventures in the previous novel (#8). Along the way they find an orphaned lion cub, which Tarzan takes home and trains.
Flora Hawkes, a previous housemaid of the Clayton’s had overheard of Tarzan’s discovery of the treasure chamber in the lost city of Opar (from The Return of Tarzan and Tarzan and the Jewels of Opar) and had managed to copy his map to it. She concocted a plan to lead an expedition to collect the gold. As a contingency to discourage any local denizens from questioning them, she sought out and found a Tarzan look-alike named Esteban Miranda to accompany them.
Two years passed since the Clayton family picked up their lion cub, making the year around 1935 and Tarzan would have been about 47 years old. His Greystoke estate had become financially depleted due his support of the Allies war efforts and he concluded it was time to return to Opar for another withdrawal.