Three cowboys help a woman in the Secret Service stop a smuggling ring trying to get poison gas to another country.
During the World War export of poison gas is banned. While with Paul Hartman (George Douglas) the horse of Ann (Doreen McKay) runs off with her. Stony Brooke (John Wayne) rescues her; but she is angry at him until Paul rides up. Paul is instructed how to get the monium for making poison gas. Judge Hastings (Joseph Forte) checks out so that Stony and Tucson Smith (Ray Corrigan) can get a room. Ann tells Frank Paige (Frank Milan) that she tried to lure Paul to him. When Paul calls on Ann, he shoots Frank and leaves. Stoney comes in to return Ann's wallet; but she calls him a thief, and he is arrested. Lullaby Joslin (Max Terhune) pretends to be a sheriff to help Stony escape. Tucson and Lullaby come in Ann's window, and she locks them in a room.
A $1,000 reward is offered for Stony. In a house Stony finds sick Frank and Ann. Frank mentions papers and dies. Ann tells Stony that she is Secret Service and enlists his help. Disguised as Ezekiel Saunders with a beard, Stony and Ann turn in Frank's body as Stony for the reward. Tucson and Lullaby read about it and go after Saunders. Ann makes Stony avoid them. Stony goes to the refinery and fights two men but is found by Paul. Stony gives him a letter he found and is accepted as one of them. Tucson and Lullaby arrive and fight men but are caught and brought to Paul and Stony, who offers to question them with two men. Stony helps Tucson and Lullaby escape but is caught and identified. Tucson and Lullaby report to Ann, and they see that Stony is tied up as the monium is transported in wagons. Tucson and Lullaby attack the last wagon and free Stony. Their wagon is chased; but they get on the horses, and the wagon burns and explodes. At the border they ambush the wagons. Ann gets the cavalry. Stony shoots the gas which burns. Wagons drive off; but Stony, Tucson, and Lullaby climb up and jump down on them, making them stop as the cavalry arrives. In the final scene Ann thanks Stony, and the three mesquiteers ride off.
This western reflects the fear that the poison gas used in the world war may be used again.