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On the road to Del Oro, Hopalong Cassidy meets Doc Bates and his daughter Paula, the propreitors of a medicine show, when he comes to their aid after their wagon breaks down. Hoppy, who has been sent to Del Oro by the United States Marshals to investigate a robbery and murder at the Gardner silver mine, buys a share in the Bates's concern and rides into town undercover as a mind reader in the show. That night, Hoppy rescues Ma Burton when her buckboard horses bolt, and she invites him to board at her house. Unkown to Hoppy, Ma is the leader of the gang of outlaws responsible for the mine robbery. When Ma begins to question Hoppy, he tells her that he is an ex-convict. To hide his real identity, Hoppy asks his friend Lucky, who has followed him to town, to pose as the marshal. Hoppy begins to suspect the kindly old lady of involvement in the mine robbery when he sees the intials of the murdered payroll carrier on a watch that Doc has lifted from the room of Blake, a known gang member. To trap Ma and her gang, Hoppy enlists them in a scheme to rob the mine, but his plans go awry when Ma overhears Lucky say Hoppy's real name and she realizes that he is a lawman. When Ma and her gang arrive at the mine, they hold up Hoppy, and after loading the silver on the buckboard, set fire to the office and leave Hoppy and Gardner to die. Meanwhile, in town, Paula overhears Ma discussing the robbery and is taken hostage by Tex Barnes, one of the gang. Back at the mine shack, Lucky rides to Hoppy's rescue, and together they apprehend Ma and her gang and rescue Paula.