Because cattle have been disappearing in the county at an alarming rate, the Red River Cattlemen's Association decides to seek government help in capturing the rustlers. In response to the cattlemen's pleas, the state attorney general sends the The Three Mesquiteers, Stony Brooke, Tucson Smith and Lullaby Joslin, members of his civilian reserve, to investigate. On the way to Red River, the Mesquiteers help Tex Reilly, an old friend who is being chased, and discover that he has been working undercover to infiltrate the rustlers' gang. The rustlers now know Tex's identity, so he and the Mesquiteers decide that he should change places with Stony and go with Lullaby and Tucson. While Tucson, Lullaby and Tex stay at a ranch owned by Pop Mason and his daughter Jane, who is in love with Tex, Stony, pretending to be an outlaw named "Killer Madigan," checks into a hotel at a dude ranch. Stony signs the register "Jack Benson," but is immediately recognized as Madigan by a bellhop who has seen a wanted poster with Stony's picture over the name Killer Madigan. A short time later, when young Tommy Jones and his father go for a ride, they are chased by the rustlers and Mr. Jones is wounded. Tommy goes to the Mason ranch to get the Mesquiteers, but by the time they return, Mr. Jones is dead. Tucson sees a large truck and tire tracks nearby and thinks that they have something to do with the disappearing cattle. Meanwhile, at the dude ranch, Stony meets Evelyn Maxwell and her mother, who "teach" him how to ride. When Tucson and Lullaby arrive to talk with Stony, they act as if they do not know one another. While pretending to get in a fight with Stony, Tucson reveals that he thinks the cattle rustling parties staged by the ranch as entertainment for the guests, are actually part of the real rustler's activities. Stony then goes on one of the rustling parties, pretending to be a greenhorn. Tucson deduces that when the guests at the ranch round up the cattle during their party, the rustlers then take them away in trucks like the ones he saw earlier. That night, Stony, posing as Madigan, asks the rustlers if he can join them, and the boss of the rustlers, Payne, who is also the manager of the hotel, gives him $500 to get rid of the Mesquiteers. The next day, word gets into the newspaper that two of the Mesquiteers have been killed. Working with Jane and Tex, Stony gets the local citizens to help by planning a big funeral for the "dead" Mesquiteers. Their plan is to have it appear as if the entire town is at the funeral, thus leaving the way open for the rustlers to make one last haul of cattle. While the rustlers are stealing the cattle, however, the Mesquiteers will be waiting to trap them at Box Canyon. Although details are supposed to be kept quiet, Mr. Randall, a member of the Cattlemen's Association, tells the town banker, Mr. Hartley, about the plan, and Hartley tells the rustlers, with whom he has been working all along. The rustlers are soon captured when Tucson, who is riding in one of the trucks with Hartley, realizes that the banker is part of the gang, and a new location is found to capture the rustlers.