Bar-20 foreman Buck Peters sends his trusted ranch hands, Hopalong Cassidy and Johnny Nelson, to deliver $15,000 to Jim Kerrigan, a friend who is having difficulty meeting his lumber contract with the Ajax Lumber Company because of recurring sabotage. Hoppy and Johnny agree to stay and help Jim and all are relieved when Jim's daughter Elaine returns home with a group of loyal lumberjacks known as "The Fighting Forty." Local lumberjacks, who refuse to work Kerrigan's camp because of numerous accidents, try to stop the Forty from working, but the Forty beat them back. After Hoppy kills one saboteur who sneaks into camp in the night, he follows the compatriots to Ajax operative Ed Petrie's office and learns of Petrie's involvement. Hoppy and Jim later decide to try and smoke out the head man behind the sabotage by staging a falling out between Jim and Hoppy during which Jim throws Hoppy and Johnny out of his camp because they supposedly cheat at cards. Petrie is now sympathetic to them and along with Preston Yates, who is in collusion with Ajax and is trying to get Jim's land, they lure Petrie and his men into attempting another sabotage. Petrie's men are captured, but Jim is severely injured during the raid, and Baptiste Deschamp, his foreman, blames Hoppy and Johnny for the sabotage. Jim revives long enough to reveal the truth of the situation, and Hoppy and the lumbermen unite forces to prevent Petrie from blowing up the dam. The lumber contract is fulfilled, the Fighting Forty leave the camp and Hoppy, Johnny and Hoppy's friend California return to the Bar-20.