Mark Lane and Reg Bachelor have been partners for some time, working as ranch hands in Wyoming for the warm part of the year, then heading south before winter sets in. In the fall of 1935, they set out as usual, traveling on horseback down into northern Arizona, where they encounter a Morman enclave. The Mormon ranchers hire them to capture the wild mustange who compete with their cattle herds for graze and water. Mustanging, however, is not easy work: it requires patience and cunning and outright grit. Thus begins a series of adventures that will lead the partners south onto the Navajo Reservation, where they are eventually hired by the representative of an eastern dog food company to trap horses in Mexico. There they settle in the tiny village of St. Theresa, bordering the great Caldern̤ land grant. But old Honorio Caldern̤ detests everything about gringos, and the mustangs have run free on his land for a long time. Flor Caldern̤, his granddaughter, is attracted to Reg and intercedes on their behalf. But that may not be enough. Gringos have been known to quietly disappear on the Caldern̤ estate...