When marshal Hammond Jr. is murdered during a bank robbery in Gila Springs, Arizona, his father Jim, head of the border marshals, sends for the vacationing Range Busters to determine who is behind his son's murder and the recent rash of robberies. The Range Busters, comprised of Dusty King, Alibi Terhune and Dave Sharpe, are angered by their friend's death and eagerly ask Jim to swear them in as marshals. When they ride into Gila Springs, Cole, head of the local vigilante group that acts as a cover for the outlaws, insists that they leave because the town has no room for strangers. The Range Busters decide to go to the ranch of Mike Rand, a friend of Jim, where they know they will be welcome, but Cole sends his underling, Mike's son Jeff, to follow them. On the way, the Range Busters rescue Mike's daughter Mary from her runaway buckboard, but she accuses them of being bandits and demands to be "freed." After they release her, Jeff fires at the marshals, and while Dave pursues Jeff, Mike's ranch hand, Tiny, takes Dusty and Alibi hostage. Jeff escapes from Dave, and after their identities are determined, the Range Busters reveal their mission to Mike. When Jeff finally comes in, Dave pretends not to know him, then sends Alibi to follow Jeff when he rides back to town. Alibi sees Jeff conferring with Cole about the presence of the Range Busters, and Cole later appears to welcome them. When Ace, a former convict incarcerated by the Range Busters and the leader of Cole's hold-up gang, picks a fight with the Range Busters, a brawl erupts in the saloon and the Range Busters sneak out carrying Jeff. At the ranch, Jeff confesses his involvement with Cole, who took over the vigilantes and corrupted the group. Mike is shocked to hear about his son's duplicity, but they all agree to work together and entrap Cole and his gang by spreading a rumor that the bank has taken in a large amount of money. Ace and the bandits attack the bank that night and are surprised by an immediate response from the Range Busters. During the ensuing gunfight, Jeff tries to prevent Cole, who killed Hammond, from doing the same to the Range Busters, but Cole knocks him down. Dusty and Alibi then round up the robbers while Dave beats up Cole, repaying him for his crimes with punches. The outlaws are arrested and the Range Busters return to vacationing.