Bob Franklin (Regis Toomey) patrols the Mexican border with help from his trusty dog, Rinty. Ed (Jack Mulhall), another patrolman, informs Bob that a criminal named El Zorro has crossed the border. Bob leaves Rinty behind while he and Ed pursue El Zorro. Meanwhile, Bob's sister arrives at his cabin. While she waits for him to return, El Zorro shows up and murders her. Enraged, Bob quits the border patrol and goes undercover to hunt down El Zorro.