Hopalong Cassidy leaves for New York, unaware of the danger of a gang of thieves led by Frazier, owner of the Devil-May-Care gold mine. Frazier's miners have been secretly mining by night a vein of the Freeze-on mine, which is adjacent to Frazier's property. When Dennis, the owner of Freeze-on, is killed in the mine following a string of unexplained fatal accidents, Lucky Jenkins, Hoppy's young partner, races to the train station and retrieves Hoppy before the train leaves. Hoppy convinces the wary Freeze-on miners to return to work and asks miner Slade, whom he suspects of duplicity, to appoint a night watchman, and Slade picks Les Martin. That night, Lucky kidnaps Martin and hides him at Buck Peters' ranch, where Lucky works, replacing him with Hoppy's old friend, Windy Halliday. While on duty, Windy feigns being both deaf and senile and sees Frazier's miners working the Freeze-on mine. Hoppy then kidnaps Dennis' murderer when he comes through Frazier's secret mine door and verifies that Frazier's ore is from the Freeze-on. Slade and Hoppy take the murderer to a deserted shack at Sandstone Gulch, but Frazier's henchman Perkins sends for help, and Hoppy is bound and gagged inside the shack. Slade sets fire to the shack, but Windy saves Hoppy, who lays low while Windy and Dennis' widow Ann get help from Buck. Hoppy then surprises Slade and Frazier in town and Buck holds them. At the mine, Windy signals to his friends, and Buck's men and Lucky then ambush Frazier's operation. Frazier escapes Buck's hold, but Hoppy chases him and knocks him out. Ann then leaves for a trip East after promising to return to share her new riches with Hoppy.