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While searching for the notorious "Polka Dot Bandit" during a raging thunderstorm, Jake, the sheriff of Yucca City, seeks refuge in a hotel.  Soon after, the Polka Dot Bandit, so called because of the distinctive bandana he wears, sneaks into the hotel and steals a $4,000 payroll from the safe.  After the bandit slips out, a man who has been sleeping surreptitiously underneath the hotel stairway inspects the crime scene and is seen by Jake through a "spy hole."

Eventually, Jake tracks the man, whose name is John Carruthers, to a remote cabin.  As Jake is about to arrest John, they both hear the cries of Betty Mason, whose father has just been killed by outlaws during a raid on the Yucca City provisions wagon, and rush to her rescue.  After a fierce gun battle, John and Jake save Betty, who then goes to stay with Malgrove, a Yucca City bigwig and the "silent" leader of the gang.

At Malgrove's ranch, Betty overhears her host and his men callously discussing her father's death.  Before she is caught and imprisoned in the ranch house, Betty also learns that, by ambushing all of the provision wagons, which bring vital supplies across the mountains, Malgrove hopes to buy out the area homesteads cheaply and make a fortune on a newly found gold vein.

Unaware of Betty's plight, John rides to Malgrove's and, while snooping in the bunkhouse, discovers that Danti, one of Malgrove's hands, is actually the Polka Dot Bandit.  After a series of fights with Malgrove's men, John is rescued by a still suspicious Jake.

That night, John returns to Malgrove's and, after locating Betty, learns of the gang's plot. John tricks the outlaws into riding to the mountain pass, while he frees Betty and uncovers the cache of stolen provisions.  With the gang at their heels, John and Jake ride toward town with the provisions and, by detonating a trail of dynamite, beat the outlaws.  Once Malgrove is exposed, John reveals to both Jake and Betty that he is a United States Marshal and not the Polka Dot Bandit.  With Betty by his side, John heads for Sacramento.