The Jack Bull tells the story of Myrl Redding, a Wyoming horse trader who clashes with Henry Ballard, a fellow rancher, after Ballard abuses two of Myrl's horses and their Crow Indian caretaker, Billy. When Judge Wilkins throws out Myrl's complaint, the war he wages to force Ballard to nurse the emaciated animals back to health escalates into a vigilante manhunt, murder and the possible defeat of Wyoming's bid for statehood.

Two young horse traders guide a Mormon wagon train to the San Juan Valley and encounter rugged terrain, the cutthroat Clegg gang, hospitable Navajo, and moral challenges on the journey.

Randall and his sidekick Manny (Frank Yaconelli) played horse traders battling a greedy and unscrupulous rival (Tom London). When some of his stock disappears, Jack follows the trail to a ranch belonging to Harriet Morgan (Marin Sais) and her young niece, Ann (Phyllis Ruth). A white stallion is accused of luring the Morgan mares astray but the horses are in reality being rustled by the ever-present London and his henchman (Charles King).