The soil of the Salinas Valley in California is rich, although the surrounding foothills are poor and life shrivels during the long dry spells. The Irish-born Hamiltons, arriving after American settlers displaced the Mexicans, settle on the barren hillside. Sam Hamilton, full of talk, glory, and improvident inventions, and Liza, his dourly religious wife, bring up their nine children there.
In Connecticut, Adam Trask and his half brother Charles grow up in harmony despite the differences in their natures. Adam is gentle and good; Charles is roughly handsome and has a streak of wild violence. After Adam’s mother commits suicide, his father marries the docile woman who becomes Charles’s mother. Adam loves his stepmother but hates his father, a rigid disciplinarian whose fanatic militarism begins with a fictitious account of his own war career and whose dream is to have a son in the army. He hopes to fulfill his dream through Adam. Charles, whose passionate love for his father goes continually unnoticed, cannot understand this rejection of himself. In despair, he beats Adam almost to death.
Adam serves in the cavalry for five years. Then, although he hates regimentation and violence, he reenlists, for he can neither accept help from his father, who became an important figure in Washington, nor return to the farm Charles now runs alone. Afterward, Adam wanders through the West and the South, serving time for vagrancy, and finally comes home to find that his father has died, making him and Charles rich. In the years that follow, the two brothers live together on the farm. Their bickering and inbred solitude drives Adam to periodic wanderings. Feeling that their life is one of pointless industry, he talks of moving west but he does not do anything about it.
Meanwhile, Cathy Ames is growing up in Massachusetts. She is born unable to comprehend goodness, but she has a sublimely innocent face and a consummate knowledge of how to manipulate and deceive others to serve her own ends. After a thwarted attempt to leave home, she burns down her parents’ house, killing them, and leaves evidence to indicate that she was murdered. She becomes the mistress of a man who runs a string of brothels and uses his love for her against him. When he realizes her true nature, he takes her to a deserted spot and beats her savagely. Near death, she crawls to the nearest house—the Trasks’—where Adam and Charles care for her. Adam thinks her innocent and beautiful; Charles, who has an empathetic knowledge of evil, wants her to leave. Cathy, who knows she temporarily needs protection, entices Adam into marrying her. On their wedding night, she gives him a sleeping draught and goes to Charles.
Aware that Charles disapproves of Cathy, Adam decides to carry out his dream of going west. He is so transfigured by his happiness that he ignores Cathy’s protests; since she is his ideal of love and purity, he thinks that she cannot disagree with him. Adam buys a ranch in the richest part of the Salinas Valley and works hard to prepare it for his wife and the child she expects. Cathy hates her pregnancy and tries unsuccessfully to abort the child. After giving birth to twin boys, she recuperates for a week; she then shoots Adam, wounding him, and leaves, abandoning her sons.
Changing her name to Kate, Cathy goes to work in a Salinas brothel. Her beauty and seeming goodness endear her to the proprietress, Faye, and Kate gradually assumes control of the establishment. After Faye makes a will leaving Kate her money and property, Kate engineers Faye’s death. Making her establishment one that arouses and caters to sadistic tastes, she becomes legendary and rich.
Adam is like a dead man for a year after his wife leaves him, unable to work his land or even to name his sons. Finally, Sam Hamilton awakens him by deliberately angering him. Sam, Adam, and Lee, the Chinese servant and a wise and good man, name the boys Caleb and Aron. The men talk of the story of Cain and Abel, and Lee declares that rejection terrifies a child and leads to guilt and to revenge. Later, after much study, Lee discovers the true meaning of the Hebrew word timshel (thou mayest) and understands that the story means in part that man can always choose to conquer evil.
Sam grows old and he knows that he will soon die. Before he leaves his ranch, he tells Adam about Kate and her cruel, destructive business. Adam visits her and suddenly knows her as she really is. Though she taunts him, telling him that Charles is the true father of his sons, and tries to seduce him, he leaves her a free and curiously exultant man. However, he is unable to bring himself to tell his sons that their mother is not dead.
Caleb and Aron grow up very differently. Golden-haired Aron inspires love without trying, but he is single-minded and unyielding; Caleb is dark and clever, a feared and respected leader whom others leave much alone. When Adam moves to town, where the schools are better, Aron falls in love with Abra Bacon, who tells him that his mother is still alive. Aron cannot believe her because to do so will destroy his faith in his father.
About this time, Adam has the idea of shipping lettuce packed in ice to New York. When the venture fails, Aron is ashamed of his father for failing publicly. Caleb vows to return the lost money to his father. As they face the problems of growing into men, Aron becomes smugly religious, which is disturbing to Abra because she feels unable to live up to his idealistic image of her. Caleb alternates between wild impulses and guilt. Learning that Kate is his mother, he begins following her until she, noticing him, invites him to her house. As he talks to her, he knows with relief that he is not like her; she feels his knowledge and hates him. Kate, obsessed by the fear that one of the old girls has discovered Faye’s murder, plots ways to destroy this menace. Although Caleb can accept Kate’s existence, he knows that Aron cannot. To get the boy away from Salinas, Caleb talks him into finishing high school in three years and beginning college. Adam, knowing nothing of Caleb’s true character, is extravagantly proud of Aron.
When World War I breaks out, Caleb goes into the bean business with Will Hamilton and makes a fortune because of food shortages. With growing excitement, he plans an elaborate presentation to his father of the money once lost in the lettuce enterprise. First he tries to persuade Aron, who seems indifferent to his father’s love, not to leave college. Caleb presents his money to Adam, only to have it rejected in anger because Adam’s idealistic nature cannot accept money made as profit from the war. He wants Caleb’s achievements to be like his brother’s. In a black mood of revenge, Caleb takes Aron to meet his mother. After her sons’ visit, Kate, who is not as disturbed by those she can hurt as she is by someone like Caleb, makes a will leaving everything to Aron. Then, burdened by age, illness, and suspiciousness, she commits suicide.
Aron, unable to face the new knowledge of his parents’ past, joins the army and goes to France. Adam does not recover from the shock of his leaving. Abra turns to Caleb, admitting that she loves him rather than Aron, whose romantic stubbornness keeps him from facing reality. When the news of Aron’s death arrives, Adam has another stroke. As he lies dying, Caleb, unable to bear his guilt any longer, tells his father of his responsibility for Aron’s enlisting and thus his death. Lee begs Adam to forgive his son. Adam weakly raises his hand in benediction and, whispering the Hebrew word timshel, dies.