Stony Brooke, Rusty Joslin and Rico, the Three Mesquiteers, are returning from Mexico and are stopped at the border by Army officials, who are attempting to apprehend smugglers who are buying cheap silver in Mexico and smuggling it into the States, where they can take advantage of a silver stabilizing measure and sell it at a high price. The Army men suspect the Mesquiteers. Don Diego, uncle of Rico and Carlos is being urged by Ransome to sell his mine but refuses. This decision conflicts with the plans of Ransome, who is the brains behind the smugglers, who are using a connection between a Ransome mine - on the Mexican side, to the Juanita mine - on the USA side. Don Diego's plans to reopen his mine lays Ransome open to exposure. Ransome has Don Diego murdered and the blame laid on Carlos. The Mesquiteers, convinced that Ransome is involved in the silver smuggling as well as the murder of Don Diego, set about to prove Carlos' innocence and Ransome's guilt.