On a secluded island south of the equator, Dolores Winthrop lives with her guardian, Blanca De Castro, a mysterious woman who refuses to reveal Dolores' parentage or explain the young woman's presence on the island. Occasionally the two women are visited by a man identified by the villagers as "the mad American." On the mainland, the grasping Don Pedro learns that the American has located a diamond mine, and in an effort to discover its whereabouts, the villain sails to the little island.