A coastal antiquarian discovers a secreted account and diagram pointing to a Spanish galleon sunk off a tropical shore, prompting a young inventor and his allies to organize a deep-sea salvage. The narrative follows their outfitting and voyage, underwater descent and clashes with rival salvors, inventive mechanical devices used in the search, and a sequence of calamities on reefs and beneath the waves. Episodes shift between technical ingenuity, tense diving encounters, parley and combat among divers, and the eventual recovery of the buried wealth and its consequences, emphasizing daring, resourcefulness, and the hazards of maritime salvage.