Our first ill-fated family is the Trasks: there's Cyrus, a brief soldier for the Union who is obsessed with the army, and his two sons, Adam and Charles. Adam is the more beloved by his father, despite being softer, but Charles craves his father's love and doesn't get it—and on one occasion he takes his anger out on Adam. Cyrus forces Adam into the cavalry, while Charles stays home and takes over the farm. After Cyrus's death, the brothers learn that Cyrus left them a suspiciously large amount of money.
By chance Adam and Charles come across Cathy, a manipulative, sexual, and (secretly) evil woman. Charles understands the kind of person she really is, but Adam builds her up into his perfect Eve. He marries her and takes her to California, where he buys a nice big ranch. This is where Adam meets Samuel Hamilton, an Irish-American farmer and intellectual with a large family, a heart of gold, and a lot of bad luck when it comes to making money. He and his family appear throughout the plot.
Cathy learns that she is pregnant, but after giving birth to twin boys she up and leaves to go be a prostitute in the town of Salinas. The realization that Cathy is a completely different person than he imagined her to be shocks Adam, and it takes him a while to recover. In the meantime, the boys, Aron and Caleb (Cal), are raised by Adam's highly-overqualified Chinese-American servant, Lee, who pretty much runs the show when he's not thinking philosophical thoughts.
The boys grow up, and eventually Adam moves the family to Salinas. Aron develops a relationship with his childhood crush, Abra, while Cal is dark, clever, and restless. Adam clearly favors Aron, who is super naïve and religious. Aron can't stand the dirtiness of the world, and he also really couldn't care less about his father's opinion of him. Cal, on the other hand, loves his father and wishes that he would love him back. Cal also learns the truth about his mother (Adam had told the boys that she was dead), but doesn't tell Aron.
When Adam loses most of his money in a failed refrigeration scheme, Cal decides to make it back and buy his father's love. He goes about it by taking advantage of the U.S.'s entry into World War I and the rising price of beans. When Aron is back home from college, Cal gives his father the money.
Far from being pleased, Adam is disappointed that Cal used the war to make a profit. In his anger and jealousy, Cal takes Aron to their mother's whorehouse (side plot: Cathy took over the house after murdering the previous madam). Aron can't handle the dirty truth, and enlists in the army. Adam has a minor stroke when he finds out.
Cal is wracked with guilt over his actions. Eventually Aron is killed in the war, and Adam suffers a major stroke that leaves him paralyzed. But Lee has Adam give Cal his fatherly blessing nonetheless, and Adam's last word represents the choice all of them have to not be re-makes of their Cain-Abel heritage.