Barbara Franklin and Rin Tin Tin are assaulted by the nefarious bandit El Zorro. Rinty is wounded and Barbara is slain while defending the canine. Blaming Rin Tin Tin for his sister's death, border patrolman Bob Franklin (Regis Toomey) sets out to get revenge. Using the information gained from a captured member of El Zorro's gang, Franklin infiltrates the smuggling ring. Through tests of loyalty Franklin must prove himself to King, the gang's boss, while he awaits his chance to meet up with El Zorro. While undercover, he befriends Ann (Molly O'Day) and John Norton (John Elliot), who are being held hostage while their home is used as a hideout. Meanwhile, the loyal Rin Tin Tin risks his life as he relentlessly protects his master.James Oliver Curwood wrote Skull And Crown and other Rin Tin Tin adventures including The Test (1935), Caryl of the Mountains (1936) and The Silver Trail (1937).