Gaucho escapes from Braden's gang only to be shot by them. The Mesquiteers drive away the outlaws and take his money on to his mother. But Isabella thinks Tucson is her long lost son and they don't have the heart to tell her he is dead.

After Bart Braden's gang robs a bank run by Samuel Tyndal, José Ojara, a gang member known as "The Gaucho," finds among the confiscated papers a mortgage for the Ojara ranch in Texas. José reminises about his mother Isabella, the "madrecita" whom he has not seen for many years, and vows to pay the $5,000 due on her ranch. Meanwhile, Braden confers with cohort Monk Stevens about a geologist's report on the Ojara ranch, which has valuable bauxite deposits. After José steals $5,000 from Braden and escapes from the gang, he meets "Stony" Brooke, "Tucson" Smith and "Lullaby" Joslin, friends who are known as The Three Mesquiteers. The Mesquiteers share their dinner with José, who tells them that he is going to Black Rock to see his "sweetheart." As the gang approaches in the distance, José rides off, and when the Mesquiteers follow to help him, he draws the gang's fire as they are about to ambush the Mesquiteers. The gang is routed, but José is fatally injured. Before he dies, José asks them to take a medal to his sweetheart, and after finding the money and the mortgage papers, the Mesquiteers decide to carry on with José's mission. Upon arriving at the Ojara ranch, the Mesquiteers are dismayed to find that Isabella believes that Tucson is José, for he is carrying the medal given to Jose by his mother. Touched by Isabella's joy at finding her long-lost son, the Mesquiteers cannot bring themselves to tell her the truth, and so go to pay off the mortgage. Tyndal is upset, for he wanted to foreclose on the ranch and obtain rights to the bauxite, but when a clerk tells him that the Mesquiteers are using money stolen in the robbery, Tyndal has the Mesquiteers arrested. After explaining that they found the money, the Mesquiteers are released, and promise to pay off the mortgage and get enough money so that Isabella can reopen the bauxite mine. Meanwhile, Braden and Stevens make a deal with Tyndal to get the Mesquiteers out of the way in exchange for a half-interest in the mine once Tyndal forecloses on Isabella's land. When the Mesquiteers arrive in town, however, they capture Braden and turn him in for a $10,000 reward. Returning to the ranch, Tucson is amazed by the appearance of little José, whom José's sister-in-law has brought to live with him. Further complications ensue when the sheriff arrives to arrest the Mesquiteers for José's murder. Tucson explains to the sheriff and Isabella that he is not José, but that the Mesquiteers did not kill him. Escaping from the sheriff, who is working with Tyndal, the Mesquiteers cash the reward check and are about to pay off the mortgage when they learn that the gang has kidnapped little José and are demanding a $10,000 ransom. Despite a trap the gang has set up, the Mesquiteers rescue little José and apprehend Tyndal, Braden and Stevens. With the mortgage paid and the mine reopened, all is well at the Ojara ranch as the Mesquiteers reassure Isabella of José's honesty and love for her, then set off in search of another adventure.