After drunken cowhand Gabe causes a cattle herd to stampede, Dave Saunders and Chito Rafferty, who are in the area looking for work, help Sandy, a sober hand, subdue the animals. Gabe's boss, rancher Fred Warren, fires the surly Gabe, and in retaliation, Gabe shoots Fred in the shoulder, then takes off. On his way to Oro City to fetch a doctor, Dave sees Gabe and leads him at gunpoint to the Warren ranch house. While Dave guards Gabe, Chito rides into town and is pleasantly surprised to discover that the doctor is a young woman named Ann F. Rollins. After the flirtatious Chito delivers Ann to Fred, Dave and Chito offer to escort Gabe to the sheriff. As Gabe is mounting his horse, however, he jumps Chito and flees before either Chito or Dave can stop him. Dave and Chito chase Gabe to the Mexican border, where they give up their pursuit. In the border town of San Lenore, Gabe runs into saloon owner Ace Kelso, a criminal acquaintance, and eagerly agrees to help Kelso and his men, Regan and Hooker, rustle Fred's herd. To accomplish the theft, Kelso has Regan, Hooker and Gabe ambush and kill Bert Grahame, a cattle inspector who is due to inspect Fred's herd. Regan then takes Grahame's identification, intending to pose as the inspector as soon as Gabe has insured that the cattle can be condemned without question. That night, while guarding the herd, the newly hired Chito and Dave catch Gabe bending over a hogtied calf and, after a fight, chase him off. The next day, Regan, posing as Grahame, has Dave rope the calf and, pointing out its swollen feet, declares that the animal is suffering from blackleg, or anthrax, a highly contagious, deadly disease. Regan condemns the entire herd and tries to leave with the animals, but Dave refuses to release them without Fred's permission. While an armed Dave guards the herd, Chito races to tell Fred the bad news. Fred and two other ranchers, Jackson and Noble, then ride to look at the herd themselves, and Noble confirms Regan's diagnosis. Although faced with ruin, Fred is compelled to turn his cattle over to Regan and Hooker, his "assistant," who lead them to Box Canyon. Unemployed once again, Chito and Dave decide to ask Ann about possible jobs and ride to Oro City. Ann is startled to hear about the blackleg outbreak and wonders whether it can be transmitted to human beings. Dave and Chito offer to ask Grahame about her concerns, but when they arrive at Box Canyon, they are surprised to see Gabe with Hooker and Regan. Suspicious, the cowboys confront Gabe, and after slugging him, Dave demands to know what he is doing. When Gabe refuses to respond, Dave lets him go, then he and Chito tail him to San Lenore. In Kelso's saloon, Kelso, who has been alerted by Gabe, starts a brawl with Chito and Dave and plants his wallet on Dave. Kelso tells the police that Dave stole his wallet, but Dave and Chito flee before being arrested. The fugitives race to the county seat to see the chief inspector, John Layton, and meet up with Ann, who informs them that Grahame refused to let her look at Fred's herd. After Dave reveals Gabe's connection to both Grahame and Kelso, Layton gives Dave orders to deliver to Grahame, forbidding him from moving the cattle until further notice. Dave and Chito find Regan in Grahame's office, who pretends to accept Layton's command, then rushes to see Kelso. Dave and Chito follow Regan to San Lenore and, outside Kelso's saloon, overhear him discussing plans with Kelso to steal the herd that night. Using a shortcut, Dave and Chito race back to Box Canyon. While Dave holds Kelso and his men at bay in the canyon, Chito enlists the aid of Jackson, Noble and Sandy. A vicious gunfight ensues, and Kelso and his men are defeated. Later, Dave shows Fred how Gabe tied the calf's legs with thin strands of horse hair to cause them to swell. Although Chito wants to stay in Oro City to romance Ann, Dave insists they move on to their next job.