In the late nineteenth century, a man named Samuel Hamilton settles in the Salinas Valley in northern California. He brings his strict but loving wife, Liza, with him from Ireland. Although Samuel is well respected in the community, he never becomes a wealthy man. The Hamiltons go on to have nine children and become a prominent family in the valley.
Adam Trask, meanwhile, settles in the valley with his pregnant wife Cathy where he eventually becomes friends with Samuel Hamilton. Before moving to California, Adam lives on a farm in Connecticut with his half-brother, Charles. The dark and moody Charles resents the fact that his and Adam’s father, Cyrus, has always favored the good-natured Adam. Upon his death, Cyrus leaves his sons a large and unexpected fortune, probably stolen during his days as an administrator in the U.S. Army. Despite their newfound wealth, Adam and Charles remain unable to get along. Charles is disgusted at his brother’s marriage to Cathy, who, unbeknownst to Adam or Charles, is a former prostitute who murdered her parents and stole their money. Although Charles despises Cathy, he takes her into his bed after she drugs Adam on their wedding night.
Adam and Cathy move to California, as Adam proves unable to live peacefully with Charles in Connecticut. In Salinas, Cathy learns she is pregnant and attempts to abort her baby in order to prevent any furtherance of ties to her husband. She is desperate to escape Adam despite the fact that he loves her and provides for her. The abortion is unsuccessful, and Cathy eventually gives birth to twins, Aron and Caleb (Cal). It is clear from the start, however, that Cathy does not care about the infants and wants to leave the household as soon as possible. One day, Cathy shoots Adam, flees the house, and moves to Salinas proper to resume her life as a prostitute. Adam decides to cover for Cathy by lying to the local sheriff and saying that his gunshot wound was an accident.
Cathy wins the trust of Faye, the madam of a local brothel, then poisons her and fools the doctors and other prostitutes into thinking that Faye died naturally. Cathy assumes control of the brothel and starts to blackmail powerful men in Salinas with photographs of them performing sadomasochistic sex acts with her and her prostitutes. To protect the dazed Adam and his twin boys, neither Samuel Hamilton nor Lee, Adam’s housekeeper, tells Adam or the boys that Cathy works at a brothel.
As the twins grow older, Aron manifests his father’s good heart, whereas Cal exhibits his mother’s ruthlessness and tendency to manipulate. By the time they reach early adolescence, however, Cal actively struggles against his dark side and prays to God to make him more like Aron. Adam, meanwhile, remains melancholy and listless for years after Cathy’s departure. In order to jolt Adam out of his despondency, Samuel finally tells him the truth about Cathy. Samuel dies soon afterward.
After Samuel’s funeral, Adam visits Cathy at the brothel. Her deteriorating body and cynical, vulgar talk make Adam realize that he can now move on and forget her, as she is a repugnant creature who has become irrelevant to his life. Cathy, however, is desperate to retain power over Adam. She even offers to have sex with him to keep him in the brothel and prove that he is no better than she. Adam refuses and leaves with a serene smile.
After his triumph over Cathy, Adam becomes a livelier and more committed father to his boys. Adam decides to move the family off the ranch and into the town of Salinas so that Aron and Cal can attend school. The twins are assigned to the seventh grade, and Aron begins a relationship with Abra, the goodhearted daughter of a corrupt county supervisor. Cal continues to struggle with his dark side, and when he finally happens to discover the truth about his mother, he believes that her evil has been passed down to him. But Adam’s housekeeper, Lee, who has extensively researched the biblical story of Cain and Abel, advises Cal that God intends each individual to choose his own moral destiny rather than be constrained by the legacy of his parents. This idea, encapsulated by the Hebrew word timshel (meaning “thou mayest”), counters Cal’s fatalistic idea that he has inherited his mother’s evil and sin.
Aron gradually withdraws into religious fervor in order to shield himself from the corruption of the world—an approach that Abra and Lee consider cowardly. Adam, meanwhile, squanders the family fortune on a poorly executed business venture involving refrigerated shipping of vegetables. Aron graduates from high school early and leaves for Stanford University. Adam misses Aron terribly, thinking him smarter and more ambitious than Cal.
However, Cal, in collaboration with Will Hamilton, one of Samuel’s sons, works secretly to earn back the fortune his father lost on the failed refrigeration business. Cal also hopes to make enough money to pay for Aron’s tuition at Stanford. In the strained economy of World War I, Will and Cal buy beans from local farmers at an unfairly low price and sell the beans, in turn, to desperate British buyers at an unfairly high price. The venture nets Cal thousands of dollars, which he plans to give to his father as a gift at Thanksgiving.
Aron, who is miserable at Stanford, comes home for Thanksgiving. Adam is thrilled to see Aron but appalled by Cal’s gift of money. Adam considers the money to be earned dishonestly and tells Cal to give it back to the farmers from whom he stole it. Enraged and jealous of Adam’s obvious preference for Aron, Cal loses control of his anger and rashly tells Aron the truth about their mother, Cathy. When Cal takes Aron to the brothel to show him that Cathy is still alive, the revelation crushes the fragile Aron, who screams incoherently and runs away. The next day, the shattered Aron joins the Army, while Cathy, horrified by her son’s reaction to her, commits suicide by overdosing on morphine. She leaves her entire fortune—part of it inherited from Charles, part of it earned through blackmail and prostitution—to Aron.
When Adam discovers that Aron has joined the Army, he lapses into a state of shock. Lee talks to Cal about the idea of timshel and urges Cal to remember that, despite his guilt, he is a normal, flawed human being—not an aberrant embodiment of evil. This discussion makes Cal feel somewhat better, and he is able to begin a relationship with Abra, who is no longer in love with Aron.
A telegram arrives informing the family that Aron has been killed in World War I. Adam has a severe stroke upon hearing the news, and Lee brings Abra and Cal to see Adam on his deathbed. Lee informs Adam that the guilt-stricken Cal told Aron about their mother only because Cal was convinced that their father loved Aron more than him. Lee asks Adam to offer his blessing to Cal before he dies. At this, Adam raises his hand and whispers the single word timshel.