Stony Brooke, Rusty Joslin and Rico, the Three Mesquiteers, are riding to Mexico to celebrate the marriage of Rico's brother Carlos when they are stopped at the border by officials who are investigating a ring smuggling silver into the United States. Unknown to the trio, the smugglers are operating by means of a tunnel that links a silver mine on each side of the border, and the Juanita mine on the Mexican side is owned by Rico's uncle, but has been closed for years. The smugglers, led by Ransome, the owner of the mining supply depot, attempt to buy the mine, but when the old man refuses to sell, Ransome orders him slain and frames Carlos for the murder. After Carlos is arrested, Ransome's men organize a lynch mob, but the Mesquiteers save Carlos from the hangman's noose and hide him in the hills. Feigning friendship for Carlos, Ransome sends Maria, Carlos' bride with a message for the fugitive, and then arranges for his men to follow her to the hideout. The Mesquiteers hold off the bushwackers while Carlos and Maria flee in her carriage, but when Carlos is wounded during the escape, Maria takes him to the army post, where he is arrested for murder. During Carlos' trial the next day, Stony sneaks into Ransome's store and discovers the entrance to a hidden mine shaft in the cellar. Meanwhile, Rico and Rusty visit the Juanita mine, where they also discover the hidden shaft housing the smuggled silver. As the Mesquiteers struggle with Ransome and his men, a keg of dynamite explodes, and in the chaos, Ransome escapes. Stony pursues him and forces a confession, thus exonerating Carlos of all guilt.