As a miner named Perez leaves his El Coronado Mine to transport some uranium ore, he is attacked by a gang of outlaws. The gang steals the uranium and pushes both Perez and the wagon into the reservoir a few hundred feet away. When the mine's new owner, Craig Bennett, tries to claim the ore against the mine's insurance company, Great Southwest Insurance, investigator Roy Rogers is assigned to the case. Roy is given the company truck, and for the ride into town, he decides to tie his horse Trigger to the back. On the way, Roy stops at a feed store, which happens to be owned by the leader of the gang, a man named Ross. Later, in town, Roy visits Frank "Doc" Harding, who is also secretly working with the gang. When Roy mentions that he has run out of money, Doc offers him a job working for Bennett at the Coronado Power and Light Company. The next day, Bennett sends his secretary, Pam Reynolds, out to the lines to introduce Roy to the rest of the crew. On her way back to the office, Pam injures her ankle while trying to move a couple of steers that have blocked her car. This is witnessed by Roy, who rides over, dismounts and begins laughing at her. Furious, Pam grabs Trigger and gallops back to the feed store. Later, Roy informs miner Sparrow Biffle of the investigation. At the mine, Roy and Sparrow see a truck leave with a second load of ore, and Sparrow says that he does not recognize either of the men inside. When they follow the truck, one of the men shoots Sparrow, hitting him in the shoulder. After Bennett learns what has happened from his men, he fires Roy. Roy then reports Perez's murder to the sheriff, and Bennett is arrested. That evening, after his Geiger counter detects some radioactive material inside Doc's desk drawer, Roy insists that Doc unlock it. As Doc is explaining that it is only his radium syringe, the gang tosses a rope across the main power lines, causing the entire valley to go dark. The blackout sends power plant employees rushing to examine the lines, and under the cover of darkness, the gang empties the mine's storehouse. Later, Roy learns that Perez's death was caused by strychnine poisoning and orders Doc at gunpoint to open his safe. When Doc pulls his own gun, Federal investigator Jim Russell, who has also been assigned to the case, suddenly shoots him. After Roy and Jim leave a note asking the sheriff to organize a posse, they ride out to the dry lake bed, where Ross and the gang wait for Doc. A plane lands, and its passenger, a man with a thick foreign accent, tries to purchase the ore from the gang. Suddenly, the posse arrives, and Ross begins climbing the electrical tower, trying frantically to escape capture. When the power is restored, Ross is electrocuted and falls to his death below.