In San Francisco's Chinatown, Arizona mine owner Manning seeks help from famed private detective Charlie Chan, as someone is trying to kill him. Manning suggests that Chan pose as a tourist and check into the Lazy-Y Dude Ranch near his "Golden Eye" mine. Chan's son Tommy and chauffeur Birmingham accompany Chan to the ranch in Arizona, and when they arrive, Chan encounters San Francisco police lieutenant Mike Ruark, who is working undercover for the government. After Chan finds that their cases may be related, Mike tells Chan that Manning has just fallen down a mine shaft and has a possible skull fracture. Mike also informs Chan that, after many years of low ouput, the Manning mine has recently become one of the richest in the country. Claiming to be a dealer in oriental curios, Chan goes to the Manning home, where he meets Manning's daughter Evelyn, mine superintendent Driscoll and his wife, and local gold assayer Talbot Bartlett. Chan also sees Manning, but he is unconscious and his head is completely swathed in bandages. Back at the ranch, Chan finds Bartlett, who is the son of an old friend and has recognized him, waiting for him. Bartlett tells Chan that he is assaying some of the gold coming from the mine. Later, Evelyn, who is unaware of the mine's recent tremendous productivity, tells Bartlett that the nursing sister, whom Driscoll has hired from the nearby mission, is strange and uncommunicative. When Pete, an independent miner, who has gained access to the mine, brings a sample of ore for assaying, Bartlett tells him it is worthless. Chan then arranges for Pete to take him into the mine through a trapdoor in his shack, but when he, Tommy and Birmingham show up, Pete is not there. However, they enter the mine and find Pete's body. After Mike tells Chan that he has learned that Driscoll has a criminal record, Chan discovers that Manning's nurse is a fake. Mike goes down the mine and is mugged, but escapes. Meanwhile, Chan finds out that Driscoll has been smuggling gold in from Mexico, where the price is much lower, and passing it off at a much higher value. Driscoll wants Manning and everyone else out of the way so that his operation can continue. When Chan, Tommy and Birmingham return to the mine, Chan finds another body with a bandaged head. They all return to the Manning house and Chan begins to remove the bandages from the patient's head. Sister Teresa rushes in, gun drawn, just as Chan reveals the patient to be Mrs. Driscoll. After Evelyn struggles with and subdues Teresa, Chan has to break the news to her that her father is dead. Driscoll enters and draws his gun, but Tommy outsmarts him and tells Chan that he has received a phone call from the Mexican police, who announce say that they have stopped the next shipment of gold at the border. Driscoll tries to escape but is shot by Bartlett, whom Chan captures and reveals to have been the brains behind the entire operation.