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.