Young adventurer, Joan Lowell, with her elderly father, Nicholas Wagner, and two crew members, ex-marine William Sawyer and Otto Siegler, sail from New York to the Caribbean in their 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 Guatemala 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 their 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 coconuts and 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. Head villager Manola is suspicious, but Maya reluctantly gives her consent but threatens Joan with death if she betrays her trust. Trailed by Manola and his men, Joan, Bill and Maya set out on the river and use the old map to locate the Mayan ruins and the temple that houses the coveted emerald. With Bill's help, Joan, blinded by her greed, diverts Maya and begins to hunt for the emerald, callously destroying a sacred goddess idol when she is caught in the act by Princess Maya. She tries to escape while Bill fights off Manola and his men but Princess Maya overtakes her and they fight fiercely until Joan overcomes her and tries to escape again. She is captured by Manola and Princess Maya sentences her to burn alive for her lies and sacrilege to the goddess idol. As the fire burns around Joan, she is rescued at the last moment by Bill. Manola and his men chase the escaping adventurers in their boat. But just as it appears they will escape back to the schooner the outboard motor stops and villagers close in to recapture them. Joan and Bill pour gasoline into the water and set fire to it to deter the villagers but the flames begin to engulf their own boat. Joan and Bill chop a hole in the bottom of the boat, dive in, and swim under the flames to safety to their schooner. Joan confesses her greed to her father and vows never to be tempted by material wealth again.