The sixth of the Hopalong Cassidy films, with the story source credited to Clarence E. Mulford's "Mesquite Jenkins, Tumbleweed", finds Hopalong Cassidy and his young pal, Johnny Nelson, leaving their Bar 20 home range to answer a letter offering them jobs on the Tumbling-L Ranch of Big John Trumbull near Yucca. Before they arrive in town, they save an old wrangler named Windy from drowning, who has been fired on from ambush as he was delivering a valuable stud bull to the depot. Windy, whose sole trusted weapon is a blacksnake whip, tells them he works for the Three-J Ranch adjoining Trumbull's spread. Hoppy and Johnny soon learn that Trumbull's outfit isn't the kind they want to work for, turn down the job offer, and take work with the Three-J, operated by easterner Jim Jordan and his sister Sally. Jordan is planning on fencing in his grazing land, but Trunbull swears this won't happen because, unknown to the other ranchers, Trumbull's men have been driving rustled cattle through a pass on the Jordan land. Hoppy and Johnny aid Jordan in getting his fence up, while Trumbull's men first try to stop them by gun warfare and then by starting a stampede of rustled cattle toward the narrow pass where the fence is being erected. Hoppy dynamites a dam, releases a torrent of water, and then succeeds in turning back the herd on the rustlers. The troubles at the Three-J are over and Hoppy and Johnny head back to the Bar 20.