"Tucson" Smith, "Stony" Brooke and "Lullaby" Joslin, friends known as The Three Mesquiteers, join the Texas Rangers, but are frustrated in their attempts to capture a notorious gang led by Val Lamar. After committing a crime, Lamar's gang leaves Texas and crosses the border into the Cherokee Strip in Oklahoma, which, as Indian territory, is out of the rangers' jurisdiction. When Lamar's younger brother Pete kills a stagecoach passenger, Lullaby moves the border sign a few feet so that Pete will be within their jurisdiction as Tucson fights with him. Pete is brought to trial, and although Lamar's threats intimidate the jury, ranger Captain Timothy Sheldon guarantees their safety and they find Pete guilty. On the day Pete is hanged, the Mesquiteers escort Sheldon's daughter Doris back to her convent school while Lamar begins taking his revenge on the jurors. After two of the jurors are killed, the Mesquiteers convince Sheldon that the rangers must raid the strip and capture the gang. Their plans are circumvented by the gang, however, who have killed the governor's nephew, James Warren, on his way to visit Sheldon. Harvard, one of Lamar's men, impersonates Warren and brings Sheldon a letter from the governor warning him not to go into the strip for any reason. The Mesquiteers grow resentful of Sheldon's adherence to the letter of the law, and after the judge is killed by Harvard, who claims that the gang did it, they decide to infiltrate the strip. Disguised as Indians, they go to the gang's saloon where they overhear Lamar making ominous plans for Doris. Harvard has forged a letter from Sheldon to Doris, requesting that she come home immediately, and so, when the Mesquiteers ride to the school to check on her, they find that Doris has already left. After the Mesquiteers find the stagecoach in which Doris is traveling, the gang attacks. Doris and Lullaby are then captured, while Tucson and Stony escape with the message that Sheldon is to come into the strip alone if Doris is to be released. While Sheldon is traveling to the strip, Tucson sees a newspaper photograph of the governor's nephew and realizes that Harvard is an impostor. Tucson and Stony then force Harvard to accompany them to the saloon in the strip, where Sheldon is about to be lynched in revenge for Pete's death. The Mesquiteers, aided by ranger reinforcements, make short work of the gang and soon receive the thanks of Sheldon and the governor. Finally, after Lullaby loses his pay to an Indian midget in a poker game, the Mesquiteers ride off in search of a new adventure.
