The story begins in the winter of the mid to late 1930s when Billy steals away in the middle of the night to watch a pack of wolves chasing antelope on the moonlit Animas Plains. The narrative then jumps forward some years with Billy and his father on the trail of a pregnant she-wolf that has wandered off the Sierra Madre mountains to prey on the Parham’s cattle. Following a prolonged hunt, Billy finally manages to trap the wolf. It is then that he makes the fateful decision to cross the international boundary line with the intention of releasing her back into the Mexican mountains.
Not long after crossing the border into Sonora, Billy encounters two Mexican horse-riders who confiscate the wolf. The wolf is taken by way of a fair to a great hacienda where it is chained within a cockfighting pit to battle dogs. Unable to release the wolf, Billy finally steps into the pit and ends the wolf’s suffering by shooting her with his father’s rifle. At dawn of the next day, Billy buries the wolf’s carcass, with the pups in utero, near the wall of the Pilares mountains.
After wandering the countryside for some time, Billy returns to the States to discover his parents murdered, his younger brother Boyd and their dog the only survivors. After picking up Boyd from a neighbour’s house, the two brothers set out for Mexico to recover the horses stolen from their ranch. They eventually regain the horse Keno and learn from a horse trading firm at Casas Grandes that their horse was part of a lot sold by the Babícora ranch. While journeying to La Babícora to retrieve the other horses, the boys rescue a young Mexican girl from two men intent on raping her.
Eventually the boys happen upon three more of the stolen horses among a remuda on the Boquillas road. After an unsuccessful attempt to retrieve the horses, the boys follow the vaqueros herding the horses toward La Babícora. Quijada, the superintendent of the Nahuerichic, retums the horses to the boys. But at noon the following day, in Boquillas y Anexas, Billy and Boyd get into a confrontation with the vaqueros that results in their leader breaking his back. Two days after, the vaqueros ambush the boys and Boyd is shot through the chest. After a retreat on the horse Nino, Billy loads Boyd into passing a truck with a group of labourers who take him to an old hacienda inhabited by campesinos. After finding a doctor to treat Boyd, Billy works with the herders while his brother continues to heal. One morning Billy goes to check on Boyd, only to find he has left with the Mexican girl.
After wandering the towns in search of his brother, Billy eventually returns to the U.S. and learns that the country is at war. He tries to enlist in Deming, El Paso and Alburquerque, but is turned away each time when a medical reveals an irregularity in his heart. Billy spends the next few years drifting through southeastern Arizona and southwestern New Mexico working at a series of ranches, before he makes his third journey into Mexico, this time to find his brother. He learns of Boyd’s death and retrieves his brother’s remains from an old cemetery at Buenaventura. He finally arrives back in America and buries Boyd in a cemetery south of Animas on Ash Wednesday.
Billy drifts aimlessly through Silver City and the surrounding towns, and ends up spending the night in an abandoned waystation where he chases off an old crippled dog. The next morning he wakes to a strange inexplicable light paling away to the east. He looks for the dog but finds no sign of it. After a while, Billy sits upon the road, holds his face in his hands and weeps.