A Greek sponge diver discovers a beautiful antiquity at the sea's bottom on one of her dives, and then she becomes obsessed with selling it via the help of her scamming boyfriend.

Off the Greek island of Hydra in the Aegean Sea, Phaedra, a beautiful peasant girl who tends the village windmill, dives for sponges with her belligerent boyfriend Rhif. Although Rhif makes empty promises to Phaedra about providing her with financial security, Phaedra's true concern is the welfare of her younger brother Niko. While prowling the undersea terrain one day, Phaedra spots a statue of a boy on a dolphin, which is tied to the mast of a sunken ship. As she swims closer to investigate, Phaedra cuts her leg on a clump of spiny coral, and Rhif takes her to the town's besotted physician, Dr. Hawkins. After extracting a nail from her wound, the doctor dismisses Phaedra's report about an undersea statue as an hallucination. The next day, however, Hawkins visits Phaedra, unfurls a drawing of the statue and exclaims that the nail he extracted from her wound belongs to an ancient vessel that sank thousands of years earlier. Sensing profits, Rhif sends Phaedra to Athens in hopes of interesting some wealthy Americans in the statue. At the Parthenon, Phaedra shows the drawing to American archaeologist James Calder. Wary of peasants claiming to have found lost treasure, Calder turns her away. Phaedra then meets Victor Parmalee, an imperious opportunist who has made his fortune in stolen antiquities. After fruitlessly trying to interest other archaeologists in the statue, Phaedra returns to the earnest Calder, who, recognizing the drawing as a mosaic found on the island of Delos, arranges to meet Phaedra later that evening at an outdoor café. Phaedra arrives early for the meeting, and is refused service because she is unescorted. Seeing Parmalee seated at a nearby table, Phaedra insists on joining him. When Parmalee learns that Phaedra is to meet Calder, his rival, he tricks her into accompanying him to his yacht. After instructing his crew to hold Phaedra prisoner until he can determine the statue's authenticity, Parmalee travels to Meteora, an isolated monastery perched upon a mountain precipice, the repository of an archive of rare manuscripts detailing the history of Greece. Upon arriving, Parmalee is surprised to see Calder seated at a table, researching the history of a statue of a gold boy seated upon a bronze dolphin. After Calder warns Parmalee that it is illegal to remove antiquities from Greece, Parmalee returns to his yacht, woos Phaedra with promises of riches and sails back to Hydra with her. Upon reaching the island, Parmalee drops anchor at a secluded cove and sends Phaedra ashore, instructing her to set a marker at the site of the statue. Calder has also landed on the island, and to avoid him, Rhif, Phaedra and Hawkins furtively sneak out to Parmalee's yacht. When he discovers that Calder is on the island, Parmalee instructs Phaedra to lead him on an unsuccesful search for the statue. Two weeks later, Niko, who has befriended Calder, points out Parmalee's yacht docked in Smuggler's Cove, and Calder realizes that Phaedra is in league with Parmalee. That night, Calder coldly informs Phaedra that he has acquired a scientific device that will help him locate the statue. Alerted by Phaedra, Parmalee directs her to remove the statue from the ship's hull and conceal it in a nearby grotto. The next morning, Phaedra is awakened by Calder, who confesses he will miss her and tenderly kisses her on the shoulder. Confused because she is falling in love with Calder, Phaedra goes to the church to pray for guidance. That night, a self-important Rhif orders Phaedra to distract Calder at the tavern while he removes the statue from the grotto and delivers it to Parmalee. When Niko informs Calder that the yacht has sailed, Calder rushes out of the tavern, followed by Phaedra. After Calder accuses her of stealing from her own people, Phaedra responds that as a rich American, he is incapable of comprehending the desperate life of the Greek peasants. Phaedra then breaks down in tears, and after they embrace, she offers to take him to the statue in the morning. The next day, they discover that the statue has disappeared, and Calder curses himself for allowing his feelings to compromise his mission. Later, Rhif slaps Phaedra unconscious and ties her up aboard his boat. At the harbor, meanwhile, Calder meets an agent of the Greek government, who informs him that he has been following Parmalee and intends to retrieve the statue. Meanwhile, Phaedra, a captive on Rhif's boat, spies Niko hiding in the hills above the harbor and signals him that the statue is suspended under the boat, secured by ropes. Niko runs to tell the doctor, and Hawkins quickly sobers up and takes charge. Just as Rhif approaches Parmalee's yacht to deliver the statue, the government boat pulls alongside and the agent informs Parmalee that he is under arrest. When they discover that the ropes have been cut, thus liberating the statue, Phaedra smiles and Parmalee admits defeat and sets sail for Monte Carlo. The government boat then transports Calder, Phaedra and Rhif back to town, but when they reach the harbor, it is deserted. Just then, the cannon at the old fort booms, and the fishing fleet, led by Niko, sails in, proudly bearing the statue. When Phaedra quietly walks away, Calder chases and tackles her, and she then smiles and kisses him.