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In a western town, Hopalong Cassidy and his ranch hands, Johnny Nelson and California Carlson, meet a stagecoach carrying archaeologist Dr. Birdsall, his niece, Jennifer Kendall, Professor Stubbs and Clay Elliott of the U.S. Mint. Birdsall has hired Hoppy and his friends to lead his expedition to the ancient desert ruins of Pueblo Grande. Before their departure, Jennifer hires Doy Kee, a Chinese cook, to accompany them. Unknown to Hoppy, the expedition is being watched both by Moy Soong, owner of a Chinese trading company, and lawyer Slade Salters, who claims to represent the Chinese community. Salters sends three cohorts to follow the expedition, and Hoppy starts to worry when a knife thrown by a Chinese man narrowly misses him. While en route to Pueblo Grande, Elliott explains to Hoppy that he has come into possession of some gold nuggets whose source he cannot identify. Elliott suspects that the Chinese community is involved because similar nuggets have been sold through Chinese shopkeepers. Later that night, someone knocks out California and causes the horses to stampede through camp, but Hoppy awakens in time to alert everyone. The next day, a wheel falls off Doy Kee's wagon and Hoppy finds evidence of sabotage. However, Birdsall refuses to abandon the project and the expedition finally reaches Pueblo Grande. While digging in the ruins, Stubbs finds a relatively new statue of Buddha buried next to some ancient remains. Hoppy warns everyone not to stray from camp, but Jennifer disobeys his order, and is surprised when Doy Kee threatens her with a gun so she will not climb above the ridge. The second time Jennifer slips away, however, she disappears behind a secret wall in the ruins. After failing to find her, Hoppy discovers that their water supply has been depleted, and that Doy Kee has been murdered and a charm left on his body. Hoppy insists that the group go to the town of Piute to get water, and when Salters learns about the expedition's activities, he plots to take over the dig for his own profit while the expedition is in town. Hoppy, meanwhile, earns Johnny's anger after he refuses to go to the sheriff about Jennifer's disappearance. Instead, Hoppy goes to the Chinese mercantile and meets with Moy Soong, who agrees to return Jennifer in exchange for Hoppy's silence. Hoppy is followed into the desert by the sheriff and his posse, who have been alerted by Johnny, and by Salters and his gang. When the posse rides up, the Chinese believe that Hoppy has broken his word, but he helps them escape to the ruins. A gunfight erupts among the ruins until the Chinese take Hoppy and his friends behind the secret wall and into a long corridor, which leads to a secret valley. Hoppy is surprised by the appearance of the lush valley, and learns that the Chinese have cultivated it for years, and have supported it with the gold they have mined adjacent to the ruins. After seeing that Jennifer is unharmed, Hoppy meets with Moy Soong, and the valley's elder, Soo Chen, who seek his advice on how to buy the property. As they are all native Californians, Hoppy explains that they simply need to stake a claim on the mine, and that the land is in the public domain. Elliott, whose only thought is of the gold, attempts to flee, but Hoppy knocks him out. The sheriff and his men then infiltrate the valley, and Moy Soong sends Hoppy to Piute to file the claim for her. Elliott again tries to escape, but is shot. Hoppy races to Piute, reaching town at the same time as Salters. After knocking Salters out in a fistfight, Hoppy files a claim on behalf of the Chinese Americans. Later, Moy Soong expresses her gratitude to Hoppy on behalf of her people, and Birdsall continues his archaeological dig.