Rancher Terry Mason's cattle are scattered by James Carson's henchmen after his refusal to join the latter's crooked Cattlemen's Protective Association. Mason's ranch-hand, Perry, is secretly working for Carson, and he blames the raid on the Russell outfit. Meanwhile, Russell, packing company owner, sends for his ace trouble-shooter, Johnny Mack, to help combat the Carson gang. When Mason accuses Russell Russell of the raid, Johnny, in pretended anger, quits Russell and, with his sidekick, Juniper, works with Mason as an investigator. Carson has his visiting niece, Valerie, befriend Mason in the hopes she will convince Mason to join the association. Johnny saves Mason's foreman, Alibi, when Carson's henchmen, led by BENDA, attempt an ambush. Johnny wounds Trigger, Carson's hired gunman, in a gun duel after killing another outlaw, Loop. Carson forges a letter to make it appear that Russell wants Johnny to wreck the association.