On a little island south of the equator Dolores Winthrop, a girl in whose veins flows American blood, lives in care of Blanca De Castro, a woman of mystery. Many times Dolores asks why the house she lives in was never finished and why she must always remain on the island, but Blanca refuses to tell her. On the mainland only a short distance from the secluded island in the little village there, the house is known as the property of the "mad American." This man, Dolores' father, lives on the mainland and disappears occasionally, returning always with very valuable rough diamonds. Don Pedro, an unscrupulous fellow, determines to learn the whereabouts of the diamond mine that he may become rich. He learns that he may find out the locality of the mine on the Winthrop island and he makes the journey there. But Don Pedro's plans do not run as smoothly as he anticipated. On the island also is Spencer Crosby, of New York, a fellow whose hobby is the hunting of bugs.