The search for missing gold takes the Hardy boys to the Yucatán Peninsula where they uncover a second mystery.
Gold bullion -- a million dollars worth -- has been stolen from the Wakefield Mint under strange circumstances. Mr. Hardy is asked to investigate but before long his life is threatened, and he asks Frank and Joe to help him. The boys fly to Zurich, Switzerland, hoping to get information at the Swiss Gold Syndicate and to find the man who has stolen a valuable ancient gold figurine from a New York museum. Their search on both counts seems futile. They return to the United States, where they uncover new clues that take them to Mexico and a breathtaking adventure at an archaeological dig in the Yucatan jungle. But the Hardys' travels lead to nothing but new doubts and nagging suspicions. And now their lives are in danger. Their adversaries are cunning, elusive, and determined to eliminate Mr. Hardy, and the boys too! Events culminate in a surprising revelation when their enemies are finally outsmarted by the Hardys. - Flyleaf.