Windy and Lucky are robbed of $20,000 in gold while they are driving the stagecoach into Glenby, and Lucky is wounded in the arm. Sheriff Martin organizes a posse to find the bandits, but they are unsuccessful, and upon Martin's return to town, he questions Lucky and Windy at the Bank of Glenby. Neither man is able to identify the robbers. Only bank president Caldwell and Martin knew about the gold shipment, which was intended as a loan for local ranchers whose mortgages are due to Nixon, the real estate agent who is threatening to claim their land. Caldwell brings in a special detective to investigate Martin, as he has failed to catch the robbers. Martin confides in his daughter Mary that Caldwell and Nixon have tried to oust him out of the sheriff's office before, and that this seems to be another ploy to do the same. Mary and her younger brother Dick ride out to find Hopalong Cassidy, who is tending his cattle, to ask for his help. Along the way they stop to water their horses at Sulphur Creek, but are chased away by a man living in the abandoned cabin nearby, who, unknown to them, is one of the bandits. By the time they bring Hoppy back to town, Caldwell's special investigator has arrived. Martin invites Hoppy, Windy and Lucky to dinner, and during the meal, they discuss the fact that Caldwell and Nixon used to be business partners, and still own all the land around town except for six ranches, on which Nixon holds the mortgage. They realize that if the ranchers are unable to pay their mortgages, Nixon would become owner of the ranches, and thereby gain the water rights to the area. The next day, Hoppy sneaks up on the bandit's hideout, and sees two of the bandits pocketing some stolen gold for themselves. Dick draws the robbers out by watering his horse nearby, and while they are occupied, Hoppy marks the bags so he can identify them later. Dick continues to watch the cabin from a safe place, while Hoppy devises a plan with Martin to capture the thieves. Martin takes a posse of men out of town, which prompts the bandits to leave their hideout and bring the gold into town. Dick forewarns Hoppy of the arrival of the bandits, while Hoppy watches Nixon from Sing Loo's restaurant. In the meantime, Windy and Lucky incapacitate the detective. Windy and Lucky see the bandits bring the gold to the side of the bank, where it is received by Nixon and Caldwell. Windy and Lucky alert Hoppy to the bandits' departure, then use the secret knock they heard the bandits use to gain entrance to the bank and hold up Nixon and Caldwell. Windy and Lucky escape to Windy's cabin, and Hoppy returns to Sing Loo's and pretends that he never left. He is captured by Nixon and Caldwell, however, and after a struggle, he and Sing Loo are tied up, while the entire gang heads out to Windy's cabin. According to plan, Dick rings the church bell to bring Martin and the posse back into town, but before he can tell them about Hoppy, one of the bandits leads them to Windy's cabin. While Windy and Lucky defend themselves against the outlaw gang, Dick frees Hoppy, who pursues the posse, ordering them to cease fire as he arrives at the cabin. Much to the surprise of Windy and Lucky, Hoppy turns them over to Martin. In the jail, Martin orders everyone to remove their guns while Caldwell identifies his bags of gold, but when the money is counted, some is discovered missing. Hoppy points the finger at the bandits, telling Caldwell that his own men stole the money while he was trying to figure out a way to get it to the bank. The entire gang is conveniently arrested right there in jail, and with the conspiracy against the ranchers cleared up, Hoppy returns to the range.