Rancher Jim Marlow arrives at the San Francisco office of banker Charles Higgins with news that he has been robbed of the $25,000 loan made to him by Higgins. Marlow had borrowed the money to build a railroad spur that would circumvent the land of Buck Collins, a greedy cattle baron who charges exorbitant tolls to allow passage across his property. Suspecting a connection between the robbery and Collins' tactics, Higgins sends for Hopalong Cassidy to investigate. Accompanied by his sidekick, Lucky Jenkins, Hoppy starts for the railhead. Along the trail, they meet Speedy MacGinnis, who takes them to a nearby mission, where Hoppy finds his old friend, Padre Jose, and is introduced to Marlow's daughter Ellen. To discover who is behind the robbery, Hoppy asks Higgins to advance another loan to Marlow, reasoning that the stage will be held up again. With Speedy's help, Hoppy robs the stage himself and is watched by two outlaws waiting in ambush. The outlaws then try to rob Hoppy, but the sheriff, tipped off anonymously by Hoppy, appears with his posse and arrests all four men. Taking advantage of a talkative deputy, Hoppy breaks out of jail and frees the outlaws, who invite him and Speedy to join them. At their hideout, Speedy surprises Dave Morgan, Collins' aide, just as he is about to divide up the $25,000. Morgan threatens Speedy, but Hoppy arrives just in time to overpower the outlaws and knock Morgan unconscious. Hoppy then announces that he plans to proceed to the mission with his prisoners, hoping that Morgan will overhear and tell Collins. At the mission, Hoppy sends Ellen to inform her father about the impending confrontation. Hoppy's plan works as Collins and his men storm the mission and are on the verge of overpowering Hoppy when Marlow and the railroad workers arrive in time to arrest the outlaws.