Mark Lane and Reg Bachelor are partners, 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, riding into northern Arizona until they encounter a Mormon enclave, where the ranchers hire them to capture the wild mustangs who compete with their cattle for graze and water.Mustanging, however, is not easy work: it requires patience, cunning and grit. Heading south, they are eventually hired by the representative of an eastern dog food company to trap horses in Mexico. But there they encounter some new problems.