Adventurer Joan Lowell, with her father, Nicholas Wagner, and two crew members, William Sawyer and Otto Siegler, sails from New York to the Caribbean in her 48-foot schooner Black Hawk . Soon after their departure, Joan and the crew battle a hurricane, which damages their mast and casts them to a shipwreck graveyard. As Bill and Otto lay claim to the mast of one of these abandoned boats, Joan and her father board an old gunrunner, where Joan discovers a one-hundred-year-old map to a lost jungle city and the hiding place of a giant sacred emerald. Afraid her superstitious sailor father will disapprove of her tampering with a dead man's belongings, Joan says nothing about her find but steers the schooner toward the lost city. Shortly afterward, however, she discovers that the boat's entire water supply was drained by the hurricane. Dying of thirst, Joan and Bill drift in a rowboat to an island where a native gives them life-saving water. When they reach the village near the lost city, Joan lies about her intentions to the local matriarch, Princess Maya, in order to obtain permission to explore. Maya reluctantly gives her consent but threatens Joan with death if she betrays her trust. Trailed by head villager Manola and his men, Joan, Bill and Maya use the old map to locate the Mayan ruins and the temple that houses the coveted emerald. With Bill's help, Joan diverts Maya and begins to scour for the emerald but is caught in the act. Captured and sentenced to burn alive for her misdeed, Joan is rescued at the last moment by Bill. As Manola and his men chase the escaping adventurers to their boat, Joan confesses her greed and vows never to be tempted by material wealth again.