Released from an Oklahoma state prison after serving four years for a manslaughter conviction, Tom Joad makes his way back to his family’s farm in Oklahoma. He meets Jim Casy, a former preacher who has given up his calling out of a belief that all life is holy—even the parts that are typically thought to be sinful—and that sacredness consists simply in endeavoring to be an equal among the people. Jim accompanies Tom to his home, only to find it—and all the surrounding farms—deserted. Muley Graves, an old neighbor, wanders by and tells the men that everyone has been “tractored” off the land. Most families, he says, including his own, have headed to California to look for work. The next morning, Tom and Jim set out for Tom’s Uncle John’s, where Muley assures them they will find the Joad clan. Upon arrival, Tom finds Ma and Pa Joad packing up the family’s few possessions. Having seen handbills advertising fruit-picking jobs in California, they envision the trip to California as their only hope of getting their lives back on track.
The journey to California in a rickety used truck is long and arduous. Grampa Joad, a feisty old man who complains bitterly that he does not want to leave his land, dies on the road shortly after the family’s departure. Dilapidated cars and trucks, loaded down with scrappy possessions, clog Highway 66: it seems the entire country is in flight to the Promised Land of California. The Joads meet Ivy and Sairy Wilson, a couple plagued with car trouble, and invite them to travel with the family. Sairy Wilson is sick and, near the California border, becomes unable to continue the journey.
As the Joads near California, they hear ominous rumors of a depleted job market. One migrant tells Pa that 20,000 people show up for every 800 jobs and that his own children have starved to death. Although the Joads press on, their first days in California prove tragic, as Granma Joad dies. The remaining family members move from one squalid camp to the next, looking in vain for work, struggling to find food, and trying desperately to hold their family together. Noah, the oldest of the Joad children, soon abandons the family, as does Connie, a young dreamer who is married to Tom’s pregnant sister, Rose of Sharon.
The Joads meet with much hostility in California. The camps are overcrowded and full of starving migrants, who are often nasty to each other. The locals are fearful and angry at the flood of newcomers, whom they derisively label “Okies.” Work is almost impossible to find or pays such a meager wage that a family’s full day’s work cannot buy a decent meal. Fearing an uprising, the large landowners do everything in their power to keep the migrants poor and dependent. While staying in a ramshackle camp known as a “Hooverville,” Tom and several men get into a heated argument with a deputy sheriff over whether workers should organize into a union. When the argument turns violent, Jim Casy knocks the sheriff unconscious and is arrested. Police officers arrive and announce their intention to burn the Hooverville to the ground.
A government-run camp proves much more hospitable to the Joads, and the family soon finds many friends and a bit of work. However, one day, while working at a pipe-laying job, Tom learns that the police are planning to stage a riot in the camp, which will allow them to shut down the facilities. By alerting and organizing the men in the camp, Tom helps to defuse the danger. Still, as pleasant as life in the government camp is, the Joads cannot survive without steady work, and they have to move on. They find employment picking fruit, but soon learn that they are earning a decent wage only because they have been hired to break a workers’ strike. Tom runs into Jim Casy who, after being released from jail, has begun organizing workers; in the process, Casy has made many enemies among the landowners. When the police hunt him down and kill him in Tom’s presence, Tom retaliates and kills a police officer.
Tom goes into hiding, while the family moves into a boxcar on a cotton farm. One day, Ruthie, the youngest Joad daughter, reveals to a girl in the camp that her brother has killed two men and is hiding nearby. Fearing for his safety, Ma Joad finds Tom and sends him away. Tom heads off to fulfill Jim’s task of organizing the migrant workers. The end of the cotton season means the end of work, and word sweeps across the land that there are no jobs to be had for three months. Rains set in and flood the land. Rose of Sharon gives birth to a stillborn child, and Ma, desperate to get her family to safety from the floods, leads them to a dry barn not far away. Here, they find a young boy kneeling over his father, who is slowly starving to death. He has not eaten for days, giving whatever food he had to his son. Realizing that Rose of Sharon is now producing milk, Ma sends the others outside, so that her daughter can nurse the dying man.