Old Lucas Beauchamp, a mixed-race tenant farmer on the old McCaslin plantation now owned by Carothers Edmonds, buries his still one night to hide the evidence of his whisky-making. George Wilkens, a negro who wants to marry Lucas's daughter Nat, has also been making whisky, and Lucas intends to have him caught and sent to prison. As he works, Lucas thinks about his ancestry: he is a descendent of Carothers McCaslin himself, through his father Turl, McCaslin's illegitimate son with his slave Tomey. Lucas thinks of himself as one of the oldest living McCaslin descendents--the only one besides Isaac who still remembers Buck and Buddy in the flesh. Digging in the old Indian mound, he finds a gold coin and realizes it may be evidence of the fortune that Buck and Buddy supposedly buried on their land. He decides to devote himself to discovering the buried treasure.
Late in the evening, Lucas goes to the big house to visit Carothers Edmonds, whom Lucas does not take seriously because he inherited his McCaslin blood from a female branch of the family tree. He tells Edmonds about George Wilkens' whisky-making operation, and Edmonds is furious. Lucas remembers when Edmonds was born--Zack Edmonds's wife died in childbirth, and Lucas's wife moved to the big house to raise the baby until Lucas angrily demanded her return. He nearly killed Zack Edmonds, who had been his playmate and friend as a boy, but the gun failed to fire. Carothers Edmonds calls the police on George Wilkens, but Wilkens and Lucas's daughter Nat move George's still to Lucas's house. The only way for Lucas to keep from going to jail is to allow Wilkens to marry Nat, who will then be unable to testify on Wilkens, who will then not be required to testify on Lucas himself.
Lucas and Wilkens begin searching for the treasure, even conning a salesman out of a metal detector to help their quest. As part of the trick to get the metal detector, Lucas uses a mule that belongs to Carothers Edmonds as collateral; Edmonds is so furious that he forbids Lucas to set foot near the house again. One day Lucas's wife, Molly, an unbelievably old-looking negro woman, comes to see Edmonds; she says she wants a divorce from Lucas because of his obsession with finding the buried treasure. Carothers Edmonds is furious with Lucas--Molly was like a mother to him. Edmonds remembers the history of the Beauchamp negroes, descended from Carothers McCaslin and wealthy because of a bequest to them in his will. As a child, Edmonds was close to Lucas's son Henry, and Edmonds remembers that his father was close to Lucas himself.
Lucas impartially agrees to the divorce, wanting to know only what it will cost him. They go to town and confront the Chancellor, but before he can grant the divorce, Lucas recants and agrees to give up the metal detector and his search for the mythical treasure. He buys Molly a sack of candy, and they return home. Lucas brings the metal detector to Carothers Edmonds and asks him to dispose of it. He says he has enough money to live on and that he thinks it isn't meant for him to find the buried money.
This weird story about the black grandson of Carothers McCaslin begins the novel's focused exploration of the themes of property, patrimony, family, and inheritance. Lucas's impersonal desire to possess the gold takes him over completely, even though he is already quite well-off financially thanks to the bequest left him by Buck and Buddy. But the gold seems to represent to Lucas some sort of franchise in the McCaslin family that he has never been able to possess--as though finding Carothers McCaslin's money would be irrevocable proof that he is a real McCaslin. The gold becomes a symbol of the position Lucas has never been able to claim as (as he believes himself to be, despite the existence of Isaac McCaslin) the rightful heir to the plantation. The only thing that can overcome Lucas's obsession with the gold is Molly, his tiny, wizened wife, whose deep importance to him is symbolized throughout the story by the fire that he has kept burning in the hearth since the day they were married.
More than any other character in Go Down, Moses, the disenfranchised Lucas is obsessed with the McCaslin family tree and with the idea that all the strength and energy of Carothers McCaslin passes down from generation to generation through male lineage. When he fights and nearly kills Zack Edmonds, he asserts that the Edmonds are "woman-born" to their position (because they are descended from Carothers McCaslin's daughter) and, hence, not as strong as those who are male-born to it, such as Lucas. He is obsessed with the figure of Carothers McCaslin, whose life he views as a challenge to his own.
Lucas's sudden, unexpected change of heart after first agreeing to the divorce indicates the intimate, personal, possibly even unconscious value he places on his own family and on his wife. The candy he buys her at the end of the book makes an immediate contrast with the buried treasure he never discovers: instead of a dramatic, massive discovery, Lucas settles for a small token of affection and familiarity.