Prospector Smokey Argyle is crossing the desert with his mules when he hears gunshots and turns his mules in their direction. Riding fast, Marshal Gene Autry passes him, heading for the waterhole. There Gene discovers that "Keg" Rollins has killed a man who stopped for water. Rollins claims that he shot the stranger in self-defense after he refused to pay for the water. Gene then prohibits Rollins from charging travelers for water because the waterhole is on government land. Later, Smokey informs Gene that he and his partner, Judd Holbrook, have discovered a source of "natural cement" on Judd's land. Although he is skeptical of Smokey's claims, Gene uses the cement to build a basin for the waterhole and is impressed by its quality. Smokey then persuades Gene to accompany him to a nearby town where a dam is about to be built. The townspeople have already hired contractor Sam Brady to build their dam, even though one of his dams previously collapsed, killing three men. As Smokey leaves the meeting, he is almost run down by a wagon driven by one of Brady's men. Shortly after, Carol Bannister, the sheriff, breaks up a fight between Gene and the driver. She then gathers a posse to serve papers to Judd, who is being forced off his land by Brady. Gene and Smokey take a short cut, but when they arrive at Judd's cabin, they discover he has been wounded. Gene and Smokey hide Judd until Carol leaves. Before he dies, Judd bequeaths the land to Smokey. Until the ownership of the land is cleared up, Gene decides to hide Judd's body so that everyone will think that he is still alive. Later, in town, Smokey sets up a demonstration of his cement, which convinces most of the dam's backers of its effectiveness. Construction is halted on the dam until the rest of the cement is checked for quality by a geologist and a mineralogist. Brady then brings in two gunmen to get rid of Gene and Smokey, but they recognize Gene as a federal marshal and refuse to shoot him. Instead, Brady decides to stop the wagons carrying the cement by causing a landslide. Smokey's wagons are destroyed and several men and horses are injured in the ensuing explosion. Gene sets a trap for Brady by sending banker Clayton Hodges a telegram signed with Judd's name and arranging a meeting outside town. The next morning, the telegraph operator shows the telegram to Carol, who sends Brady to the meeting to make sure that Judd is killed. Gene stops Brady from killing Hodges and turns him and his men over to Carol. Later, however, she releases the criminals, and Gene summons his deputies to recapture them. The men and Carol are arrested, and Gene returns home.