Mr. Hackett, the general manager of the Cattlemen's Protective Association in Buena Ventura, has stolen cattle from over a dozen ranches and exhausted the association's funds by paying ranchers for their losses. Meanwhile, The Three Mesquiteers, Stony Brook, Tucson Smith and Lullaby Joslin, trustees to the Sam Clarke estate which runs the association, become suspicious because not one head of cattle has been retrieved by Hackett's men. The association has also voted to close the city's orphanage, which was endowed by Clarke and is run by Doris Moore. Doris has three days to get enough money to keep the orphanage open and to save the children from mean-spirited townspeople who only want to adopt them for house labor. To help Doris, the Mesquiteers call off all adoptions and look for proof of Hackett's duplicity. By trailing the herd of Hackett's latest victim, Wilson, the Mesquiteers chase off the rustlers and drive the cattle back to Wilson. Observing that Hackett's posse is strangely unwilling to shoot at the rustlers, the Mesquiteers confront Hackett in town and remove him and his deputies from office. When Lullaby learns that the sizable Carson ranch is Hackett's next target, the Mesquiteers hurry to the ranch, where Hackett's men shoot at them until they are forced to retreat when a young orphan named Bobby discovers them. To keep Bobby quiet, Hackett tries to adopt him, but Lullaby saves him, and the boy identifies Captain Gardner as the head of Hackett's gang. The final showdown occurs during the orphanage fundraiser, at which Hackett plans to shoot the Mesquiteers while they are on stage. In his act, however, Stony pretends to have mystic powers and forces a confession from one of the gang members, whom Hackett then shoots. In the mayhem that follows, the outlaws escape and ride to Hackett's ranch. There Lullaby pulls a gun on Hackett as he collects the pilfered association money. With the criminals caught, Stony secures the future of the orphanage and promises to visit Doris there in the moonlight. Lullaby then adopts Bobby and the Three Mesquiteers--now four--ride off.