A stagecoach carrying the Billings Mining Co. payroll is attacked when the guards are distracted by a boy being carried off in an apparently runaway wagon. Also attempting to help the boy are The Three Mesquiteers, "Stony" Brooke, "Tucson" Smith and "Lullaby" Joslin. The boy, Tim Hardin, easily gains control of the wagon, eludes the Mesquiteers and joins the gang, which is led by his father. While Hardin and his son hide the money, the town sheriff, Ed Stoddard, is castigated by the citizens for letting another robbery occur. Ed tells them that he has sent for the Mesquiteers, but Stony, Tucson and Lullaby are ridiculed themselves after they are deputized, when everyone learns that they unwittingly aided Tim. When one of the gang arrives in town to seek treatment for a wound received during the holdup, the Mesquiteers follow him to the gang's hideout. A shootout ensues, during which Hardin is shot accidentally by the wounded man. The Mesquiteers capture Tim, who accuses them of killing his father. Mistakenly believing that the Mesquiteers are mistreating Tim when they question him about the money, reporter Sue Dayton has him sent to a work farm. The Mesquiteers then lie to the farm's superintendent, Boyle, telling him that Lullaby and his wife, who does not exist, wish to adopt a son. To escape a bully, Tim accompanies the Mesquiteers to their ranch, and there he is gradually swayed by their genuine interest in him. Tim is perplexed, for his father had told him that a "squealer" was "so low he could walk under a dead skunk standing up," yet he wants to help the Mesquiteers retrieve the money as thanks for their kindness. The boy solves his problem by leaving an "anonymous" note and the receipt for the money in the Mesquiteers' mailbox, but when he returns to the house, he hears his friends arguing with Sue and Boyle, who have uncovered the adoption deception. Tim misinterprets the argument to mean that the Mesquiteers are just using him, when in fact they really want to adopt him. The youngster runs away but is caught by Taggart, who makes him retrieve the receipt. When Lullaby sees Tim by the mailbox, Taggart shoots him, and Tim leaves his note in Lullaby's jacket. Lullaby revives when Stony and Tucson return from searching for Tim, and they follow the note's instructions to a nearby Wells Fargo office, where Tim and Taggart have already retrieved the money. As Tim and Taggart make their escape in the stage, Taggart is surprised by another member of the gang, whom he was attempting to doublecross. Taggart shoots the man, then the stage's driver and guard. The Mesquiteers chase the coach, and as Tim jumps out onto Lullaby's horse, Stony subdues Taggart and Tucson stops the stage. Tim hands the money over to the Mesquiteers, and soon all ends well as they send their young ward off to military school.
