Bob Franklin, Border Patrolman on the Mexican border, is preparing for the homecoming of his sister Barbara from a fashionable school. His dog Rinty is even gathering flowers in anticipation of her arrival. Border Patrolman Ed drops by to tell Bob that Zorro, a notorious smuggler,has just crossed the line with a cargo of contraband. Leaving Rinty at the railroad station at the wheel of his car with a note of regret to Barbara, Bob and Ed hit the trail to catch Zorro. They capture the truck and driver, Matt Brent, but Zorro escapes afoot. Meanwhile, Barbara has arrived and she and Rinty have driven to Bob's cabin --- no, Rinty didn't drive --- and Zorro comes along and decides driving is better than walking and, bent on getting the keys, enters the cabin. Although valiantly defended by Rinty, Barbara is murdered after Rinty has been stunned by Zorro's gun.Bob arrives and, unaware that Rinty had put up a good fight before being knocked out, kicks Rinty out in contempt and anger. Bob resigns from the service and masquerading as a smuggler named Rocky Morgan and friend of the captured Matt, he goes to "Skull and Crown", a fashionable resort owned by John Norton, who, together with his daughter Ann are held there as prisoners by Zorro and his gang who have commandeered the lodge. Faithful Rinty, hurt feelings aside, has not renounced his devotion to Bob and has followed him there, and is promptly captured. Henchman Jennings who was with Zorro at the cabin and recognizes the dog, and knowing that Rinty will go to his master, lines up all of the gang including the newly-inducted Bob, with instructions to another henchmen---evidently the only one he trusted---to shoot down the first man the dog recognizes. Well, Rinty has seen this before (or had been told about it by Rin-Tin-Tin the Elder) and he snubs everybody, especially Bob. Meanwhile, the Border Patrol has purposely released Matt in order to follow him to the hideout and then loses him and Matt shows up and blows the whistle on Bob. Bob is imprisoned, awaiting the arrival of Zorro who has a rule against killing anybody when he isn't around to take part and, worse of all, Rinty is all tied up and can't help his master.