The threat of war dominates headlines throughout the United States, as the Neutrality Act is signed, making the sale of war-related substances such as monium illegal. Among the foreign agents smuggling monium is Paul Hartman, who is posing as a guest at a Mesquite County dude ranch. When he and fellow guest Ann go riding, Ann deliberately causes her horse to bolt, hoping that Paul will catch her. The Three Mesquiteers, Stony Brooke, Tucson Smith and Lullaby Joslin, who happen to be riding by, see the runaway horse and stop it, but receive Ann's ire instead of thanks. After she feigns a fainting spell, she rides off with Paul, leaving her purse, which the Mesquiteers discover contains a small gun. They then go to the ranch, which is run by Paul's cohort, Judge Hastings, to return the purse. Ann, who is actually a Secret Service agent, discusses Paul in her room with fellow agent Frank when Paul arrives. After a struggle, Frank kills Paul, but is wounded himself and barely escapes. Hearing the shots, Stony rushes to the room, but when others arrive, Ann says that Stony was trying to rob her and killed Paul in the attempt. Lullaby pretends to be the sheriff and "arrests" Stony, until the real Sheriff Johnson arrives and Stony barely escapes. While reward posters are circulated for Stony, he hides and comes upon a cabin in which Ann is nursing Frank. After Frank dies, she reveals her identity and Stony decides to trade clothes with Frank and, with Ann's help, pose as hillbilly Ezeckial Saunders and collect the reward for Stony Brooke. Tucson and Lullaby hear about Stony's "death" and go to claim the body. Meanwhile, Stony has assumed Paul's identity so that he can meet with Paul's contact, Gordon. Tucson and Lullaby follow him and, when caught, pretend not to know Stony and offer to join the gang. When Judge Harris arrives, he recognizes Stony, who is then locked in a wagon with the monium as the agents escape. Just before the border, Lullaby, Tucson and Ann, who had been following the wagons, stop them, and detain them until a border division of the U.S. Cavalry arrives. The Three Mesquiteers later ride away as Ann goes back to her job as a government agent.