Biff Brewster receives a 16th birthday present to go to Brazil to be with his father. Biff believes he will be going on a safari. Before leaving, Biff receives a secret letter from Mr. Stannart of the Ajax Company to give to his father. Stannart informs Biff that his father is not looking for rubber plantations, as many at the Company believe, but instead is looking for a secret gold mine. As Biff quickly discovers, others are also looking for the secret gold mine and will stop at nothing to find it. In addition to thieves, Native headhunters and wild rivers await Biff and his father.

Andy Adams is the Grosset & Dunlap pseudonym for three writers who authored the Biff Brewster series of adventure and mystery novels for adolescent boys in the early and mid-1960s. Walter B. Gibson, Edward Pastore, and Peter Harkins were real-world authors.