In 1909, near the Mexico-United States border, Martha McCandles runs a massive ranch with the help of her sons Jeff, Michael, and James. The Fain Gang (The Fain Brothers, the Devries Brothers, John Goodfellow, Kid Duffy, Breed O’Brien, Pop Dawson, and Trooper) attacks the ranch, brutally slaying many members of the staff. Jeff kills the Devries brothers, but is badly wounded; his son, Jacob “Little Jake” McCandles, is kidnapped before the gang flees to Mexico.
Martha places a million dollar ransom in a strongbox, and delegates from both the United States Army and the Texas Rangers offer to take the box for her. Martha decides instead to send for her estranged husband Jacob “Big Jake” McCandles, who wanders the west as a gunfighter with his black Rough Collie mix, simply named “Dog”. Jake arrives and they confer in secret about what to do with the box. Michael McCandles, the youngest son, arrives on a motorcycle with news he has found the kidnappers. Martha decides to allow him and his older brother James to set off with the Rangers in REO Runabouts to try to overtake the kidnappers. Jake disapproves, and sets off with the box, a mule, packhorses, and his elderly Apache friend Sam Sharpnose, preferring to do things the old fashioned way.
The kidnappers ambush the Rangers, putting the cars out of commission, and Jake allows his two sons to accompany him. Relations are strained between Jake and James after the former’s long absence from home, but Michael is delighted to see his father again and impresses him with his skill as a sharpshooter. However, Jake is put off by Michael’s more modern and genteel ways.
John Fain, pretending to be a messenger boy, intercepts the group and warns them bandits are now after the box. He tells them the gang will kill Little Jake if Big Jake – himself pretending to be a hired hand – doesn’t do things exactly the gang’s way. The family checks into a hotel and lays a trap for the bandits, killing them. During the attack, the strongbox is opened, revealing the money has been replaced by newspaper clippings. Both boys believe Jake has stolen the money, until Jake reminds them of the people that were killed by the Fain Gang, and that their brother Jeff has also been badly and possibly fatally wounded. Martha, refusing to pay for that, took the money out herself.
Pop Dawson arrives with a message to meet the gang with the money in an old fort outside town. He tells them the rules – they are to ride together, and not try anything until after the gang has left with the money, because their own sharpshooter Duffy is hidden far away with a rifle trained on Little Jake at all times. Big Jake convinces Dawson that Michael was killed by the bandits, so he, James, Sam, and Dog follow Dawson to the fort. Fain reveals he is the ringleader, and reiterates Dawson’s threats.
Jake throws the key to Fain while Michael gets into position atop the fort with his rifle. Just as Fain opens the chest and realizes the deception, Big Jake opens fire and kills John’s brother Will, freeing Little Jake. Michael takes out Duffy, Sam kills Trooper, and James shoots down Dawson and O’Brien but breaks his hand. Fain and Big Jake find themselves both wounded and in a stalemate from behind their respective hiding places, and Big Jake tells his grandson to run to James. Goodfellow slashes Sam and Dog with his machete, killing the Indian, and pursues Little Jake into a stable, where Dog attacks Goodfellow a second time and is killed. Big Jake, out of bullets, makes a run for it and kills Goodfellow with a pitchfork in the stables. John Fain corners the weaponless grandfather and grandson outside the stable, but Michael, having come down from his perch, shoots Fain from behind. As Fain dies, Big Jake finally reveals his true identity to him, and to Little Jake, who has never met his grandfather before. Reunited at last, the family acknowledges their renewed bond and prepares to go home.