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In Africa, where the Nyaga Ratu River meets the sea, an island tribe known as the Aquatanians dives for pearls, fishes and worships Balu, a false god created by the power-hungry high priest Palanth.  Balu appears to the Aquatanians as a large, mute figure who resides in a temple on top of a rocky island, which Palanth has declared off-limits to anyone but himself.  In reality, the "god" is Varga, a greedy trader in a plaster costume, who covets the pearls that the tribe gives him as tribute.

As Mara, a beautiful Aquatanian who has been chosen to be Balu's bride, is being escorted to her "groom" during her wedding ceremony, she sees through the deception and flees in horror.  Mara, who loves the banished Tiko, dives into the bay and heads upstream, eventually coming to the jungle home of wild man Tarzan and his mate Jane.  Seeing the mermaid-like Mara swimming near his river fishing spot, Tarzan jumps in and chases her through the water until she collapses from exhaustion.  Tarzan takes Mara to Jane and Benji, a singing postal carrier, who nurse her until she is well enough to tell her story.

Out of gratitude, Mara gives Jane and Tarzan a large black pearl, which Jane then decides to donate to the area's new reform-minded commissioner.  While Benji takes the pearl to the commissioner in Nyaga, Palanth sends some men to search for Mara.  Eventually, Mara is captured, but Tarzan is able to follow the boat carrying her back to Aquatania.

After Mara is returned to the tribe, Tarzan sneaks into Balu's temple and spies Varga shedding his costume and leaving the island.  Benji, meanwhile, has shown the inspector-general and the commissioner the pearl and guides the suspicious commissioner to Tarzan's home.  There Jane, who has met up with the lovelorn Tiko, tells the commissioner Mara's story, and she, Tiko, Benji and the commissioner then leave for Aquatania.

As they near the island, Palanth orders that they be brought to the temple and there begins to question them.  To Palanth's surprise, Balu suddenly appears and silently commands that Mara be freed and Tiko, forgiven.  As Balu orders that the "intruders" be allowed to leave unharmed, Jane realizes that Tarzan is inside the costume.  Unable to expose Tarzan, Palanth is forced to release the group, which then prepares to celebrate Mara and Tiko's wedding.

In Nyaga, meanwhile, Varga is questioned by the inspector-general and hurries back to Aquatania.  During Mara and Tiko's pre-nuptial celebration, Varga suddenly appears as Balu and, through Palanth, orders that the intruders be killed.  Tarzan, however, is able to fend off Varga's thugs underwater and, after fighting an octopus, swims back to the temple and exposes Balu as a fake.  The angry Aquatanians then dispose of Varga and Palanth and resume their happy celebration.