Joe Bass (Burt Lancaster), an American fur trapper, is making his way back home with a full cargo of hides when he encounters a group of Kiowa Indians led by Two Crows (Armando Silvestre), who insist on taking his furs. As "payment" they offer him a slave, Joseph Lee (Ossie Davis), who they had previously taken from a group of Comanches. Reluctantly, realising he cannot fight them, Bass agrees to the trade.
Bass is slightly bemused by his new companion, who is a well-educated and refined house slave, unused to the ways of the West. Bass orders him to help him recover the furs from the Kiowa. Lee only agrees to help if Bass agrees to take him to Mexico where slavery is outlawed. Bass is not amused, but the two strike up a friendly relatonship, with Bass teaching Lee how to catch food in the West.
As Lee and Bass come upon the Kiowa, they watch them being ambushed by a group of scalphunters, White Americans and Mexicans who were offered a bounty by the government for each Native scalp they brought in. These scalphunters, led by Jim Howie (Telly Savalas), kill the Kiowa and also steal the furs which Bass had hoped to recover. He and Lee are forced to track the party, hoping to steal back the furs.
Unfortunately, while spying on them, Lee tips over a cliff and is captured by the scalphunters who decide they can sell him for $1500 in Galveston, Texas. As they travel southwards, Jim Howie's girlfriend, Kate (Shelley Winters), reveals to Lee that they are heading for Mexico. He begins to win her favor, by doing her hair and telling her fortune, hoping she will persuade Howes to take him with them across the border, rather than sell him.
Lee meanwhile has been following closely on their trail, and at one point he pins them down with sniper fire, forcing them to let loose the packhorse carrying the furs. He is ambushed, however, and the scalphunters recover the furs and proceed on their way. Approaching their camp at night, Bass tries to persuade Lee to help him, but the slave is now set on going to Mexico and refuses him assistance.
Bass kills several of the scalphunters, by rolling rocks on them. Now thoroughly fed up, and wanting to make it to Mexico in safety, they send Lee as a courier to Bass telling him he can keep the furs. A wary Bass comes down to collect the lone packhorse, and is ambushed by Howie who had always intended to break the deal. In an ensuing struggle he is shot by Lee.
The bickering Bass and Lee then break into a long fight, in which neither can defeat the other. Meanwhile, a group of Kiowa attack and overrun the scalphunters. Two Crows, who had survived an earlier massacre, has fetched reinforcements. He takes back the furs, per their earlier deal. The story ends with Bass and Lee, now friends, preparing to follow the Kiowa to steal back the furs.