Tucson Smith and Lullaby Joslin, two of The Three Mesquiteers, are visited by Tucson's younger brother Larry while their friend and the third Mesquiteer, Stony Brooke, is in Mexico. Their adventure begins when John Evans, the inspector for the State Agricultural Service, sends them an emergency message. They rush to Evans' ranch, where he lives with his daughter Anne and son Mickey, and are told by Evans that two cows have been found dead of hoof and mouth disease and with their brands destroyed. Evans orders Tucson, his deputy, to watch the state line while he searches for the culprit, and to lift the quarantine by four o'clock if he has not reported back. At the quarantine station, Tucson, Lullaby and Larry inspect the animals crossing the border, and the delay irritates rancher Tom Brent and his foreman, Luke, who are transporting a prize thoroughbred horse. Luke gets through with the car, and while Brent waits with the horse, he offers to buy Larry's horse. Larry refuses his offer, but becomes interested in Brent's dog, Buck, after he sees Brent abusing him. Larry offers to race his horse against Brent's, the winner getting to keep the animal he desires. Lullaby slyly moves the halfway marker, and Larry wins the race and becomes Buck's owner. Luke returns to the quarantine line and tells Brent, the owner of the diseased cattle, that Evans has discovered six more sick cattle on their ranch. Brent intercepts Evans on the trail and shoots him in the back when Evans refuses to be bribed to keep silent. Two weeks pass as Tucson looks after the grieving Evans family and searches for the killer. More bad news comes, however, when Tucson finds Brent's six cattle, with their brands changed to Evans', on the ranch, and he is forced to order the destruction of Evans' livestock. Anne is crushed by Tucson's decision and angrily states that she never wants to see him again. Larry also accuses Tucson of having a badge instead of a heart, and he joins Brent at a meeting of local cattlemen. Larry agrees to drive his cattle with Brent's to a buyer across the border before Tucson posts guards at the quarantine lines. Larry leaves after Brent assures him that his cattle are healthy, after which Lullaby overhears Brent and Luke discuss having put the diseased cattle on Evans' ranch. Lullaby catches up with Larry as he joins Brent's cattle drive and informs him of Brent's deception. Larry then sends Lullaby for help while he holds off Brent and Luke, but when Luke and Larry fight, they fall down a cliff and are trapped under the debris of a rockslide. After Buck finds Larry, Larry sends him to summon help. Buck travels the dangerous route along the quarantine line, and although he is shot on the way, he reaches Lullaby and Tucson. Luke and Larry make their peace while waiting for help, and Luke reveals that Brent shot Evans. Help arrives too late for Luke, but Larry is rescued by his brother. Brent falls into a river after being shot by Larry, and the other desperados are rounded up. Soon, the hoof and mouth epidemic has been checked and the state promises to replace all cattle lost. Tucson and Anne then make up while Buck and Larry convalesce.