In an attempt to steal thousands of horses from a Navajo tribe in Arizona, outlaw Clint Bolling pretends to be a government inspector assigned to weed out the tribe's diseased animals. Bolling's forged documents allow him and his gang access to the herd, which is led by the magnificent stallion "Rex." While Bolling trains his horse, "Marquis," to lead the horses into a trap, the Indians hold a high council to celebrate the coming of age of Red Wolf. Big Man, Red Wolf's father, was once the most celebrated member of the tribe, and he decides that in order to prove his manhood, his son must capture and tame Rex. Red Wolf trails Rex into the desert and finally captures him in quicksand. Red Wolf becomes obsessed with training the beautiful animal, and even forgets Wanima, the beautiful maiden who loves him. Desperate to get rid of Rex so that Marquis can take control of the herd, Bolling goes to the corral under the pretense of inspecting Rex's health, although he really intends to kill him. Bolling torments the spirited animal until Rex, filled with hatred, escapes in pursuit of Bolling after the impostor is forced to leave the corral. News arrives from Washington that Bolling is a fake, and the tribe also searches for him. As they arrive at his hiding place, Marquis is leading the horses into a trap, but Rex suddenly appears. After a desperate fight with Marquis, Rex kills the other horse and sends his herd to safety. Rex then finds Bolling cowering in a crevice and tramples him with his hooves. Red Wolf proudly ties Rex beside Wanima's hogan, thereby indicating that the couple is engaged, and Wanima's father and Big Man begin bargaining over whether the dowry shall be forty or fifty sheep.