About a year previous to the time this story opens, these three boys had been on a trip along the Rio Grande, when they fell in with Capt. June Peak and a company of Texas Rangers, who had been detailed to keep watch of the actions of a band of cattle smugglers. Sent across the river into Mexican territory on a secret mission, the Broncho Rider Boys had the good fortune to rescue Pedro Sanchez, the fourth member of the quartette, from the hands of a band of ruffians. Pedro turned out to be the son of Gen. Sanchez of the Mexican army, who was visiting an uncle in northern Mexico. After a series of thrilling adventures, which are told in full in The Broncho Rider Boys with the Texas Rangers, Donald, Adrian and Billie returned to their homes, promising to visit Pedro in the City of Mexico whenever the time was ripe. During the time that the boys were scouting in Mexico they had learned to speak Spanish quite well, and this knowledge had been so improved during their visit with Pedro that they now spoke the language well, an accomplishment which proved of much value to them later on. About a month prior to the day upon which this story opens, the three Americans had met by appointment at New York City and had come to Vera Cruz by boat and thence to the City of Mexico, where they found everything in a greatly disturbed condition because of the revolution which had been started some months previous by Gen. Carranza. It might be well right here to state briefly the history of the previous few months in Mexico, so that all may understand how it happened that none of the four boys had a very high opinion of Gen. Huerta, at that time dictator of Mexico. For nearly 35 years, up to 1911, Mexico had a peaceful existence under a republican form of government. During the last 32 years of that time Porfirio Diaz was president. Just prior to 1912 a revolution was begun against what had come to be called the Diaz government, and Diaz was compelled to flee from Mexico. The revolution was headed by Francisco Madero, who was then made president. In February of 1913 a revolution was started against President Madero by Felix Diaz, nephew of Porfirio Diaz, and the City of Mexico was attacked. At that time Gen. Huerta was in command of Madero's forces in the City of Mexico. He proved a traitor to Madero, went over to Diaz, arrested Madero and confined him in prison. Two days later, April 22, 1913, President Madero was shot by order of Huerta, who then declared himself dictator. At the same time he asked that the other nations of the earth recognize him as the head of the Mexican government, a thing which the government of the United States refused to do. March 26, 1913, another revolution was started, this time against Gen. Huerta by Gen. Carranza, governor of the state of Chihuahua. This revolution had been in progress more than a year when this story opens.