Frog Millhouse meets cowboy Sunset Carson on the trail to Placer City while delivering mining goods to businessman J. B. Murdock. The supplies are stolen enroute, however, by Ned Crane and Marjorie Malloy, who take them to the mining shafts of Round Nugget Hill. Ned encourages the miners there to rebuild the dangerously fragile mines for their own profit and resist Murdock's interference. Murdock, a short-tempered, bullying man, is incensed when Frog and Sunset report the theft. Along with Dr. Lee and the local sheriff, Murdock, Frog and Sunset ride out to the mines to accuse Ned and Marjorie, who escape into a tunnel. Murdock, Frog and Marjorie are trapped together in a cave-in, and as water pours in on them rapidly, a panicked Murdock promises to drop the charges against Marjorie and Ned if he is saved. Ned risks dynamiting the tunnel and the three are saved, proving Ned's theory that he can restore the mines to safety with proper irrigation and gain a franchise on them. When Murdock reneges on his promise to drop charges against Ned, Frog and Sunset refuse to identify him as the supply thief. Dr. Lee, who is in on Murdock's plans to take over the mines, advises Murdock to have his henchman, Hansen, steal Ned's plans for restoring the mines. On their way to the capital to secure the franchise, Ned and Marjorie are ambushed, but Frog and Sunset rescue them, then escort the couple to the capital, where Ned's plans are approved. Later, in Placer City, Murdock tries to raise doubt about Ned's project, and to regain the miners' confidence, Frog disguises himself as a French engineer who is to oversee Ned's drainage plans for Round Nugget Hill. Lee then convinces Murdock to finance Ned's plans, taking the mines as collateral, while the doctor secretly sabotages the project by poisoning the water supply. After several men grow ill while working on the project, the miners begin to quit. Suspecting that the French engineer and Frog are one and the same, Lee arranges for Hansen to cause a phony explosion in the mine shafts that they can blame on Frog. Hansen panics, however, when Frog sees him setting up the dynamite and flees to Murdock, who pays him off and tells him to hide out. Murdock then arranges for another of his henchmen to go after Hansen and kill him. Sunset and Frog also pursue Hansen, hoping to find out who is behind the mine sabotage, but are unable to get to him before Murdock's man has shot and severely wounded him. Despite his injuries Hansen implicates Murdock before collapsing, and Sunset and Frog take him to Lee. Frog goes after Murdock, who confidently assumes the doctor will "take care" of Hansen. Sunset remains suspicious, however, and forces Lee to treat Hansen, who recovers sufficiently to indict the doctor. Lee and Murdock are arrested, and as Placer City and the mines boom, Ned and Marjorie plan to marry while Sunset and Frog hit the trail.