In 1902, President Theodore Roosevelt signs the Reclamation Act that will provide for the irrigation of vast Western wastelands, thereby making them habitable for settlers. Texas cattlemen resent the act, for they mistakenly believe that the homesteaders will steal their rangeland. After Clay Allison is appointed the District Supervisor of Irrigation for Texas, he goes to Wood River, where trouble is brewing between the established cattlemen and the arriving settlers. Allison pretends to be on the side of the homesteaders, but is actually in league with ruthless cattlemen led by Belle Starkey and Duke Plummer, the manager of Belle's saloon. The gang intends to further the range war, drive the settlers out and offer the land for sale at a later date. Local cattlemen Stony Brooke, Rusty Joslin and Renaldo, who are known as The Three Mesquiteers, are sympathetic to the settlers, expecially after meeting newcomers Kansas Jones, his lovely daughter June and son Tim. The Mesquiteers help June obtain a job with Jefferson Morgan, the outspoken editor of the Wood River Gazette who supports the Act. When Allison arrives in town, the settlers demand the dam construction jobs promised to them. Allison stalls them, and the Mesquiteers invite everyone to a party at their ranch, but their neighborly action backfires when Allison's men raid the homesteaders' empty cabins. Kansas, June and Tim have not yet left for the party, and Tim is seriously wounded by the raiders. Kansas carries his injured son to the Mesquiteers' ranch and bitterly blames them for the incident. As time passes, the gang attacks various cattlemen, making it seem as if the homesteaders are retaliating. The Mesquiteers realize that there must be someone fomenting the war for personal gain, and they devise a plan to infiltrate the gang. Stony spends three days gambling at Belle's saloon and pretends to have a falling out with his partners over it. Meanwhile, the gang attacks a stagecoach carrying the engineers who are to begin the dam project and substitute Allison's men, who will help delay it. The delay threatens the homesteaders, who are approaching starvation. The gang further tries to menace the settlers with violence, but on each occasion Stony finds out and, wearing a mask, drives away the marauders. Allison contrives to indefinitely postpone the construction, but Morgan succeeds in securing a $100,000 relief fund for the settlers from the governor. Duke invites Stony to join the gang in robbing the stagecoach that will carry the money, and after Stony alerts his friends, the Mesquiteers foil the robbery. Stony, in his guise of the masked rider, takes the money to town and tells the citizens that Allison is behind their troubles. Allison rips off Stony's mask, and the people refuse to believe Stony because he is a cattleman. Just then, Rusty and Renaldo arrive with the real engineers, and the perpetrators are all rounded up after a battle. Later, Morgan is appointed the new supervisor, and as he begins the project, the Mesquiteers travel to New Mexico to help out a friend.