A group of boys set out on an ocean voyage in a newly acquired yacht and, after rescuing a castaway, learn of a rumored Spanish treasure hidden in mountain caves off Puerto Rico. Their expedition alternates between seafaring crises—storms, a shipboard fire, and rescues—and overland hazards: jungle and cave exploration, encounters with wild animals, a mountain hurricane, a mysterious chamber of bones, and betrayal that leads to capture and a daring rescue. Teamwork, practical resourcefulness, and steady perseverance guide their search, which ultimately leads to the recovery of the lost treasure and a return voyage.