East of Eden is the story of two families, the Hamilton family and the Trask family, both of whom migrate to the Salinas Valley in California. The Hamiltons are a poor family living on dry, barren land, but Adam Trask and his wife Cathy Trask are rich from inheritance and live on a fertile and rich piece of land with a large quantity of water available. But the Trasks material wealth is undermined by their moral poverty. Cathy Trask was born without a conscience, and though Adam loves her, he is blind to her evil. She only marries him because it is convenient for her at the time, but after she bears him two sons she shoots him in the shoulder and leaves.
Catherine changes her name to Kate and works her way up in a whorehouse in the city of Salinas. She manipulates the owner of the whorehouse, Faye, so effectively that Faye wills the house and all of her belongings to Catherine. Catherine then slowly poisons Faye and, when she dies, takes over the business. She becomes the owner of the most depraved and degenerate whorehouse in all of Salinas.
After Catherine leaves him, Adam goes dead inside, going a year without giving his newly born sons names. Sam Hamilton and Adam’s Chinese-American servant Lee are responsible for Adam’s rehabilitation. Sam literally beats sense into Adam, who, as if awakened from a long sleep, begs Lee and Sam for help getting to know his children. They consider naming the twins “Cain and Abel” but after discussing the story of the Biblical Adam’s sons at length they opt not to, instead calling the boys Caleb and Aaron.
As they grow older Caleb asks to be called Cal and Aron drops the extra “a” from his name. Aron is a good boy, beautiful and blonde like his mother, and beloved by everyone. Cal is darker, and at a young age realizes he has cruel impulses, and prays to God not to make him mean. He finds out his mother is alive when he is only about ten years old while eavesdropping, but keeps this secret from Adam, who believes his mother is dead and buried on the east coast.
One day Sam, who has grown old and more bold in his old age, tells Adam that Cathy is in a whorehouse in Salinas. Shortly after Sam’s death, Adam goes to see Cathy, and when he does he realizes she has no hold over him anymore. His joy makes her furious and he tells her she is not fully human, and hates the good in people because she cannot understand it.
Adam and his sons move to Salinas, where the boys attend a larger public school. Aron begins courting Abra Bacon, a girl he plans to marry. They are still only children however, and Aron asks Abra to pretend to be his mother, and cries in her lap when she agrees. Cal grows into a lonely and somewhat troubled boy. One night he ventures out with a drunk and sees what goes on in his mother’s whorehouse. He knows that if Aron ever found out about Catherine, it would destroy him. In the meantime, Adam becomes interested in refrigeration, and devises a plan to ship fresh lettuce across the country in a train. Due largely to bad luck, the project fails, and Adam has wasted almost his whole fortune on the enterprise.
Cal decides he will earn his father’s money back, and partners with Will Hamilton to profit from the need for imperishable foodstuffs during the war by contracting for beans. Cal saves up the money while Aron graduates from high school early and goes off to Stanford. Adam is immensely proud of Aron and Cal hopes his father will be proud of him, too. When Aron is home for Thanksgiving, Cal gives his father the money he has earned from his venture, but his father is disgusted by the gift, saying he doesn’t want money earned from the cheating of farmers and a bloody war. That night, consumed by anger, Cal shows Aron the whorehouse belonging to Catherine. The next day, Aron lies about his age to enlist in the army.
While he is gone, Abra and Cal fall in love—Abra thinks her relationship with Aron was a sham, for he had fallen in love with an idealized version of her, not the real her. She loves Cal because Cal is both good and bad, and Abra knows she is both good and bad, too. Adam’s health begins to fail, and when news of Aron’s death arrives, Adam has a stroke. Despite this tragedy, the novel ends on a redeeming note—with Lee’s help, Cal obtains his father’s blessing and rids himself of the guilt of causing his brother’s death.