South of Sonora: Bill Tracy is a Cattleman's Protective Association agent. Working undercover on his way to Sonora, a town dominated by rustlers, saves Betty Carter from a situation, but her rancher father, believing Blackie's suggestion that Bill is a rustler, forbids her to see him. Bill and his pal Heinie Schmaltz rescue each other from rustlers' various plots and ambushes and eventually discover that Blackie, Carter's foreman, is the leader of the rustlers.