Don, Smokey and Whopper are are their way to Placer City where Don's brother Brad is U.S. Marshal. But when Don gets there, he finds that Marshal Brad is not his brother, but an impostor. When the deputy accuses them of being horse thieves, they flee town. When they go out to find Jenny, who they saved the day before, they are shot at and accused as gold thieves. But Jenny sets the miners straight and the boys, along with Mr. Robinson of the stage line, agree to work together to stop the gold thefts and find the real Marshal Brad.

Don Cardigan and his pals, Whopper and Smokey, are riding to the mining town of Placerville to visit Don's brother Brad when they see a speeding stagecoach driven by a drunken driver. The three race to stop the stage and rescue its passenger, Jenny Blanchard, a fussy spinster. Don offers to drive Jenny into town, where she introduces him to Robinson, the manager of the stage line. After promising to visit Jenny, Don goes to meet Brad, the town marshal. At the marshal's office, Don is shocked to find a stranger masquerading as his brother. Suspicious of the newcomer, the marshal tells his deputies to investigate. After the deputies accuse Don, Whopper and Smokey of horse thievery, the three gallop out of town toward the Blanchard house. Along the trail, they are stopped by Ben Blanchard and his daughter Penny, who think that they are robbers who have been stealing the miners' gold. When Jenny vouches for their integrity, Don tells the family that the town marshal is an impostor. Realizing that the marshal must be behind the robberies, Ben and the others visit Robinson, and Don proposes a plan to protect the next shipment. Vander, Robinson's assistant and a confederate of the marshal, eavesdrops on their conversation and informs the marshal of Don's plans to re- route the gold by packhorse. That night, Vander switches the gold-laden bags with sacks of sand and leaves town with the gold on the next stage. Don, Smokey and Whopper load the bags on the horses, but when they camp for the night, one of the bags springs a leak and they realize that they are transporting sand and not gold. When the marhsal's gang attacks them and demands the gold, Don lures them into camp with a promise to turn over the worthless sacks. After running off with the gang's horses, Don and his friends escape and ride back to town, where they learn that Vander has taken the gold on the stage. They apprehend Vander on the stage, and he confesses that the marshal's name is Reynolds and offers to take them to the shack where Brad is being held. Robinson volunteers to drive the gold to safety, and Don stops by the Blanchards' to tell them that the gold shipment is safe. Relieved, Ben offers to gather the miners and meet Don at the shack. Meanwhile, Reynolds' men arrive at the marshal's office, and upon discovering that they have carried wothless bags of sand, they ride to the shack. Don, Whopper and Smokey beat them to the shack and free Brad just as Reynolds and his men arrive. Reynolds tries to smoke them out with a burning wagon, but Ben and the miners ride to the rescue and apprehend the gang. With his brother reinstated as marshal, Don and his pals return to their ranch.