While growing up in California's Salinas Valley, the narrator learns to tell East from the bright Gabilan Mountains, and West from the dark Santa Lucias Mountains. Although the weather is cyclic - years of heavy rainfall followed by drought - the changes nevertheless surprise the Valley's inhabitants. The area was settled first by Native Americans, who were followed by the Spanish, and finally by the Americans. The narrator's grandparents, Samuel Hamilton and Liza Hamilton (named for Steinbeck's own grandparents), emigrate from Ireland in 1870 and are forced for financial reasons to settle on the Valley's most barren land, where they work hard to raise their nine children. Samuel's good nature and work ethic and Liza's good-heart endear them to their neighbors, especially the Trasks.
After moving from New England, Adam Trask settles in a far more fertile section of the Salinas Valley. As a child, he lived with Cyrus, his Civil War veteran father, his timid stepmother, Alice, and his cruel half-brother, Charles. His deeply devout mother committed suicide upon learning that Cyrus had infected her with syphilis. Cyrus also lied about his military record and moved to Washington D.C. to take a high-ranking government position. The boys had an extremely difficult childhood, but while Adam remains passive, Charles is aggressive, and beats his bother severely after Adam defeats him in a simple game. One day, Adam discovers his stepmother standing by herself and smiling, and begins to leave her presents that she believes come from Charles. His father attempts to convince Adam to join the Army, but thinks that his other son's nature is too inherently dark for such a career. Charles becomes deeply resentful when Cyrus ignores the knife he has given him for his birthday, preferring the puppy given to him by Adam. In a jealous rage, Charles attacks Adam and leaves him unconscious on the road. While Adam recuperates, Cyrus enlists him in the Army.
A self-educated immigrant from Ireland, Samuel Hamilton initially inspires suspicion amongst his new neighbors. However, in time he wins them all over with his warmth, wit, and practical know-how. Samuel and Liza Hamilton never acquire wealth, but they are rich in other ways: they have nine loving children, and a wonderful home life. Liza, the level-headed mother, works from sun-up to sun-down. She pretends to be tough in an effort to control her impractical inventor husband and her large brood of children, but she has a kind heart.
While the Hamiltons are working hard settling in the Salinas Valley, the Trask brothers remain in the East. After the savage beating from Charles, Adam joins the army, while his brother remains on the family farm. Their father, Cyrus, moves to Washington D.C. to take a high-level job in the Office of the Secretary of War. While his brother Adam is absent, Charles suffers a farm accident which leaves him with a dark brown scar on his face. Embarrassed by his scar and reticent by nature, Charles becomes even more reclusive, and longs for Adam's return. However, Adam finds the idea of returning to the farm unpalatable and re-enlists in the Army. At this point, his father sends for him, and Adam finds Cyrus living luxuriously in Washington D.C. with a new prosthetic leg. His father attempts to convince Adam to enroll at West Point Academy, but Adam refuses. Once more, Adam finds that he cannot return home after his second stint in the Army expires, and so he wanders around the country as a vagabond until he is arrested and placed on a chain gang. He writes to Charles to inform him that he has escaped, and asks him to send money so that he can return home. Upon his arrival, he learns that Cyrus has died and left the brothers a fortune of $100,000. Charles tells Adam that their father lied about his war record, and suggests that their inheritance is ill-gotten. Adam suggests that they move to California.
In chapter eight, Cathy Ames takes center stage. She has the face of an angel, but the temperament of the devil. Her parents, solid middle-class folk, adore their only child and cannot come to terms with her evil nature. The girl has an uncanny, almost preternatural ability to sexually attract men. Indeed, when she is only twelve she seduces a group of teenage boys and arranges for her mother to discover them just to see the boys suffer the consequences. Later, one of Cathy's teachers commits suicide after a mysterious liaison with her. Cathy has an intense hatred for her parents that seems wholly unfounded, but rather than just running away, she chooses to burn down their house, and they die in the flames. The neighbors are left thinking Cathy has also perished in the fire.
John Steinbeck, who casts himself as the young narrator, carefully creates the near-mythical setting in which the characters of East of Eden reside. His setting, however, does far more than merely serve as the backdrop for the action. Indeed, the setting creates thematic tension and opposition. Steinbeck sets up the biblical metaphor of good versus evil (or light versus dark) - the novel's central theme - by utilizing Salinas Valley, California, where he lived as a youngster, as the novel's primary setting. "Evil", or darkness, is represented by the Santa Lucias Mountains, which lie to the West, while "good", or light, is represented by the Gabilan Mountains, to the East. It is here, between these two mountain ranges, that the Hamilton and Trask families settle. Although the eleven-member Hamilton family is certainly fertile, they are forced to live on the most barren land in the Valley, while the dysfunctional, almost infertile Trask family lives on the Valley's richest land. This discrepancy is a physical manifestation of the fact that the strong immigrants, Samuel and Liza Hamilton, are forced to eke out a living on next to nothing, while the rich but weak Trasks, who inherited their wealth through dishonest means, seem to be sliding into degeneracy. Both the Hamiltons and the Trasks are led by heavily bearded men (Samuel and Cyrus, respectively), but the men are very different types of patriarchs: Samuel is the loving, healthy, "good" father, while Cyrus is the hateful, diseased, "bad" father. The kindly Samuel provides Adam with a positive paternal role model, but the "sins" of Adam's biological father nevertheless wreak havoc upon him and his brother, Charles.
Adam and Charles are representations of the biblical Cain and Abel, as evidenced both by their initials and their family circumstances. In the Bible, Abel's sacrifice of a lamb pleases God more than Cain's gift of grain, and Cain murders his brother in a jealous rage. God curses Cain and sends him out into the land of Nod, which lies to the East of Eden (the location referred to in the title of Steinbeck's novel). Like Cain, Charles becomes deeply jealous when their father, Cyrus, ignores Charles' expensive gift in favor of the free mongrel puppy given to him by his favored son, Adam. In a rage, Charles attacks Adam and leaves him unconscious.
The biblical tale of Cain and Abel informs the plot of the entire novel. Elements of the story are found in two generations of Trasks: Charles and Adam, and Adam's twin sons, Cal and Aron. In addition, the story goes on to be discussed for years by Adam, Lee, and Samuel Hamilton, as the three men attempt to come to terms with life's ongoing battle between good and evil. The scar resulting from Charles' farm accident is emblematic of an element in the Cain and Abel story. As part of God's punishment for Abel's murder, God places a similar scar on Cain as a warning for others to avoid him, and curses him to wander the earth for the rest of his days. In East of Eden, Charles is the one with the scar, while it is his brother Adam who wanders the earth, first as an officer in the army, and later as a vagabond. Adam eventually returns home, marries Cathy Ames, and moves to California.
In addition to the Cain and Abel paradigm, East of Eden (along with many of John Steinbeck's other novels) is replete with biblical allusions. Of particular note is the concept of original sin, as found in Genesis. Just as Adam and Eve committed the original sin by disobeying God and eating the forbidden apple in the Garden of Eden, Cyrus, the father of Charles and Adam, commits the novel's original sin by lying about his war record and acquiring money through false pretenses. Likewise, just as the descendants of Adam and Eve were cursed for the sins of their forebears, so too are the sins of the father visited upon Cyrus' descendents. Despite their large inheritance, Charles and Adam are reclusive and miserable.
Cathy Ames, whose "aims" are evil, is a representative of the Biblical antihero, Satan. Manipulative and uncaring, she seduces people into obeying her wishes. This character has been criticized by scholars for not seeming "real" enough; indeed, she fails to demonstrate even a single positive characteristic to counteract her evil traits, and at no point in this long novel does the reader ever feel one iota of sympathy towards her. Steinbeck never offers an explanation for why she burns her parents to death, seduces teenage boys, or compels her teacher to commit suicide. For Cathy, evil simply comes naturally. Later in the novel, Samuel Hamilton comes to realize that Cathy lacks some essential human element.