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In the opening chapter of the Mascot serial The Phantom Empire, a stagecoach is held up for its content -- musical instruments. One of the "outlaws," a bashful young man, removes a fake mustache to address his radio audience: "Yes folks, it's Gene Autry making another raid on your time with his 'Radio Riders,' broadcasting from Radio Ranch." After a rendition of Smiley Burnette's "Uncle Noah's Ark", the airwaves are given over to teenagers Frankie (Frankie Darro) and Betsy Baxter (Betsy Ross King), who relay the story of how they founded the "Thunder Riders' Club" for kids. Out riding in the desert one day, Frankie and Betsy spotted a group of strangely garbed horsemen who seemingly disappeared without a trace. Using old buckets as headgear, the kids' reenact the encounter and invite boys and girls to form their own groups and come visit a real radio show being produced. Unbeknownst to Gene and his radio performers, however, there is radium in them thar hills, and not only that; the ranch may also be situated on top of a secret treasure trove, the underground world of Murania, a "Scientific City" inhabited 100,000 years ago by people fleeing an ice age glacier. Professors Beetson (J. Frank Glendon) and Cooper (Edward Peil, Sr.) are after both the radium and the treasure and the former make a couple of unsuccessful attempts at Autry's life. The villains trace the radium to a place known as The Garden of Life, a development that greatly endangers the lives of the Muranian people 25,000 feet below. Their ruler, Queen Tika (Dorothy Christy), decrees the entrance to the Garden of Life be destroyed for good and the capture of Gene Autry, less the area be overrun by radio fans. Alerted to danger by the sound of the Thunder Riders, Frankie and Betsy discover a lost Gene in the desert. "Say, we have a broadcast at 2 o'clock," the rustic crooner exclaims. "If we miss it, we'll lose our contract!" Climbing down from the Garden of Life plateau, Gene, Frankie and Betsy can only watch as their rope is torn asunder by the Riders.