Hopalong Cassidy, Lucky Jenkins, and Windy Halliday work for the Bar 20 Ranch owned by Buck Peters. Buck's niece Laura Peters and his nephew Artie Peters also live there, and Lucky is in love with Laura.
Buck needs cash and hands Hopalong a difficult assignment: bring a herd of cattle to Fort Hastings, with a hard deadline of ten days to satisfy the contract. To meet the schedule, the cattle must be driven through the Black Butte area, a place notorious from the previous year when rustlers stampeded the Bar 20 herd, causing heavy losses and the deaths of several of Hopalong's men. Nora Blake, who lives in Black Butte, catches Hopalong's eye, and he finds himself drawn to her. The rustler plot thickens when the rustler chief is revealed to be Professor Hepburne, a man Hopalong encounters before the drive when the Professor stops by Bar 20 to have his horse reshod. Windy notices that the Professor is carrying dynamite, a sign that trouble is looming.
The cattle drive begins under pressure. Buck’s nephew begs to join, and Buck relents, sending Artie along to ride after the drive and later place him at the chuck wagon with Windy. As the herd moves through Black Butte, Professor Hepburne triggers the dynamite charge, and a devastating stampede follows. Two hundred cattle are lost to the attack, and rustlers threaten Hopalong’s men once again. Windy and Artie end up right beneath the blast zone and survive only by taking cover under the chuck wagon, though Artie is badly hurt in the chaos. A piece of the dynamite casing lands near Windy, confirming Hopalong’s suspicion that Hepburne is the mastermind behind the raid.
Determined to end the threat, Hopalong lays a careful trap. He places Lucky in charge of the drive, sending the cattle toward Fort Hastings while promising to return with the cash and reinforcements to confront the rustlers at the Butte. Hopalong then seeks out Nora, hoping to keep her away from the danger—and to keep Hepburne from discovering the true reach of her brother’s villainy. Windy, who has been tending to Artie, goes to fetch water and uncovers the rustlers and the scattered cattle; upon his return, Artie—unsafely determined to help—slips away from Windy to summon reinforcements back at the ranch.
The plan comes together as Hopalong, Lucky, Windy, and the ranch hands close in on the rustlers. In a furious confrontation, Hepburne’s temper flares when one of his own men speaks back; he shoots the dissenting man, and that act triggers chaos as the shooter collapses Hepburne with a retaliatory shot. The gunfight ignites the dynamite stash, and Hepburne is killed in the explosion. With the rustlers defeated and the danger behind them, Hopalong believes that Nora will never learn the full extent of her brother’s villainy. In the end, virtue triumphs over malice, and the Bar 20 Ranch can look forward to a safer future.
