Upon receiving reports that strange unclassified creatures, possibly mermaids, have been observed in the coastal waters off Tiburon Island, marine biologist Samuel Jamison agrees to co-finance an expedition to the island. His partner, Ernst Steinhauer, is a pearl trader who claims there is a rich supply of fire pearls to be found at Tiburon. When Steinhauer disappears, Jamison makes the trip alone, unaware that Milo Sangster, a fellow passenger on the chartered boat, is a ruthless adventurer who has murdered Steinhauer in the hope of getting the fire pearls for himself. Once at the desolate island, Jamison encounters a mermaid queen who leads him to an underwater cave filled with fire pearls. Sangster also locates the cave, but mermaids put him under their spell and lure him to his death. Jamison tries to find the mermaid queen to persuade her to return to civilization with him. The mermaid, realizing that she could never leave her world beneath the sea, hides from view until he reboards his ship and pulls anchor. As the vessel leaves the island, the mermaid surfaces, sadly watches it depart, and then disappears from sight.