 The sixth of the Hopalong Cassidy films, with the story source credited to Clarence E. Mulford s "Mesquite Jenkins, Tumbleweed", finds Hopalong Cassidy (William Boyd) and his young pal, Johnny Nelson (Jimmy Ellison), leaving their Bar 20 home range to answer a letter oferring them jobs on the Tumbling-L Ranch of Big John Trumbull (Sidney Blackmer) near Yucca. Before they arrive in town, they save an old wrangler named Windy (George Hayes, long before there was a George "Gabby" Hayes) 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 (Charles Martin) and his sister Sally (Lynn Gabriel.) 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.