When Julie Peters secretly marries lumberman Ben Jordan at her father's Bar-20 ranch, Hopalong Cassidy, the ranch foreman, advises her to tell her father Buck about the wedding. After Ben steps outside the house, gun shots ring out. Running to the doorway, Hoppy sees Ben's dead body and observes his murderers galloping away. After shooting one of the killers in self-defense, Hoppy discovers that the man has stolen Ben's satchell. Inside he finds an invoice from the tax collector of Pine Rock, notifying Ben that unless his overdue taxes are paid by June 1, the Jordan lands will be sold at auction. Buck, who was wounded in the attack, can identify Ben's killer, but is unable to travel because of his injuries. Since Buck is injured, Hoppy and ranch hand Jimmy Rogers offer to accompany Julie to Pine Rock. Arriving just as the tax collector is auctioning off the Jordan property, Hoppy protests the sale, and after Julie pays the delinquent taxes, the deed is returned to her. At the auction, Jimmy meets his old friend, Big Joe Williams, a homesteader who had worked on the Jordan timberlands. Also there is California Carlson, another of the Bar-20 cowhands. Fenwick, a crooked land developer bent on seizing the Jordan's rich timberlands, convinces Julie to sign a contract to supply wooden ties to the railroad and offers to assemble a logging crew to fill the order. Although Hoppy cautions Julie about becoming involved with Fenwick and apprises her that Fenwick and his partner Keeper have been evicting helpless settlers, she ignores his warning and signs the contract. Julie then orders Hoppy to return to the Bar-20, but when Jimmy recognizes Keeper's foreman, Taggart, as the man who shot Ben, Hoppy decides to stay in Pine Rock to investigate. Knowing that only Buck can positively identify Ben's murderer, Hoppy sends Jimmy to the Bar-20 to fetch him. Fenwick, meanwhile, puts Taggart in charge of the logging crew and instructs him to work slowly so that Julie will miss her contract deadline. When Julie requests an extension, Fenwick refuses and informs her that a clause in the contract dictates that she must forfeit her land if she fails to meet the deadline. Desperate, Julie rides to the lumber camp to urge the crew to work faster. When she finds Taggart deliberately destroying a cart, she strikes him with her whip and he hits her. Witnessing the attack, Hoppy rides to Julie's rescue and takes on Taggart's entire crew. Just as Taggart is about to club Hoppy, California appears, rifle in hand, and escorts Hoppy and Julie to safety. At the lumber camp, Hoppy explains to Julie that Ben owed the squatters money for working his lands. Big Joe then steps forward and volunteers to work on a new lumber crew composed of squatters. Led by Hoppy, the crew briskly begins work. Realizing that with Hoppy acting as foreman, Julie will easily meet her deadline, Fenwick orders Taggart to sabotage the crew's efforts. When California learns that Fenwick has armed his men for a shootout, he warns the loggers to take cover. Soon after, Jimmy and Buck arrive with reinforcements from the Bar-20 and join the frey. After Buck positively identifies Taggart as Ben's killer, Fenwick grabs a bundle of dynamite and heads for the dam to blow it up. As Hoppy pursues Fenwick, Jimmy apprehends Keeper. At the dam, Fenwick ignites the fuse and then takes cover in a nearby shack. Grabbing the dynamite, Hoppy throws it into the woods and it explodes at the base of a tree, sending the tree crashing into the shack, thus killing Fenwick. Indebted to the squatters, Julie presents them with trust deeds to the Jordan land, and Hoppy, his mission accomplished, leads the boys back to the Bar-20.