John Steinbeck's novel The Grapes of Wrath tells the specific story of the Joad family, and thus illustrates the hardships and oppression suffered by migrant laborers during the Great Depression. It is an explicitly political piece of writing, one that champions collective action by the lower classes. In taking this social stance, Steinbeck's novel criticizes shortsighted self-interest and chastises corporate and banking elites for profit-maximizing policies that ultimately forced farmers into destitution and even starvation.
The novel begins with a description of the conditions in Dust Bowl Oklahoma that ruined crops and instigated massive foreclosures on farmland. No specific characters emerge initially; this is a technique that Steinbeck will employ several times in the book, posing descriptions of events in a large social context against descriptions of events more particular to the Joad family.
Tom Joad, a man not yet thirty, approaches a diner dressed in spotless, somewhat formal clothing. He hitches a ride with a truck driver, who presses Tom for information until Tom finally reveals that he was just released from McAlester prison, where he served four years for murdering a man during a fight. Steinbeck follows this exchange with an interlude describing a turtle crossing the road, which serves as a metaphor for the struggles of the working class.
On his travels home, Tom meets a onetime preacher, Jim Casy, a talkative man gripped by doubts over religious teachings and the presence of sin. He gave up the ministry after realizing that he found little wrong with the sexual liaisons he had with the women in his congregation. Casy espouses the view that what is holy in human nature comes not from a distant God, but from people themselves. Steinbeck contrasts Tom's return with the arrival of bank representatives to evict the tenant farmers. The possibility of a working class insurrection is raised, but an effective target for collective action cannot be found.
Tom and Casy reach the Joads' house, only to find that it has been deserted. Muley Graves, a local elderly man who may not be sane, tells them that the Joads have been evicted, and that the family now stays with Uncle John. Muley's own family has left to find work in California, but Muley decided to stay himself.
Steinbeck then provides a description of the tactics that car dealers use to exploit impoverished customers. The dealers find that they can make greater profits by selling damaged jalopies than by selling dependable new cars.
Tom Joad finds the rest of his family staying with Uncle John, a morose man who has been prone to depression since the death of his wife several years earlier. Yet Tom's mother is a strong, sturdy woman who is the moral center of family life. His brother, Noah, may have been brain damaged during childbirth, while his sister, Rose of Sharon (called Rosasharn by the family) is recently married and pregnant. Her husband, Connie Rivers, has dreams of studying radios. Tom's younger brother, Al, is only sixteen. This introduction to Steinbeck's characters is followed by a more general description of the sale of items by impoverished families who intend to leave Oklahoma for California, as the Joads expect to do.
The Joads plan to go to California on account of flyers advertising work in the California fields. These flyers, as Steinbeck will soon reveal, are fraudulent advertisements meant to draw more workers than necessary and drive down wages. Jim Casy asks to accompany the Joads to California so that he can work with the people in the fields rather than preach at them. Before the family leaves, Grampa Joad declares his refusal to go, but the family gives him medicine to knock him unconscious and takes him along. The subsequent chapters describe the vacant houses that remain after the Oklahoma farmers have left for work elsewhere, as well as the conditions on Route 66, the highway that stretches from Oklahoma to Bakersfield, California.
Almost immediately after the journey begins, the Joad family loses two members. The first victim is the family dog, which is run over during the Joads' first stop. The second is Grampa Joad, who dies of a stroke. The Wilson family helps the Joads when Grampa dies, and the two families decide to make the journey to California together. Steinbeck follows this with a larger statement about the growing of a collective consciousness among the working class, who shift their perceptions from "I" to "we."
The Wilsons' car soon breaks down, and Tom and Casy consider separating from the rest of the family temporarily to fix the car, but Ma Joad refuses to let the family break apart even for a short time. Tom and Al do find the necessary part to fix the car at a junkyard. Before the Joads set out on their journey again, they find a man returning from California, who tells them that there is no work there and that the promises of work in the flyers are all fraudulent.
The Joads and Wilsons reach California, where they are immediately subjected to intimidation by police officers who derisively call them, and other migrant laborers, "Okies." At the first camp where the Joads stay, Granma becomes quite ill, but receives some comfort from proselytizing Jehovites who merely annoy Ma Joad. The next time the police stop the Joads on their travels, Ma Joad forces the authorities to let the family pass without inspection. She does this to conceal the fact that Granma has died.
Steinbeck follows this with a description of the history of California, which has apparently been marked by oppression and slavery. However, he predicts an imminent revolution, for the people there have been deprived to such a great degree that they must at some point take whatever they need in order to survive.
At the next camp where the Joads stay in their search for work, they learn about Weedpatch, a government camp where the residents are spared harassment by police officers and have access to amenities such as baths and toilets. Later, when police officers attempt to start a fight with Tom and several other migrant workers, Tom trips an officer and Casy knocks him unconscious. To keep Tom from taking the blame (and from consequently being sent back to jail for violating his parole), Casy accepts responsibility for the crime and is taken away to jail. The rest of the family begins to break apart as well. Uncle John leaves to get drunk, Noah decides to leave society altogether and live alone in the woodlands, and Connie abandons his pregnant wife. Before they must move on, Tom does retrieve Uncle John, who is still consumed with guilt over his wife's death. They head north toward the government camp.
At the government camp, the Joads are shocked to find how well the other residents treat them and how efficiently this society (which even features democratic elections) functions. Tom also finds work quickly, but the contractor, Mr. Thomas, warns him that there will be trouble at the dance at Weedpatch that weekend. Since the police can only enter the camp if there is trouble, they intend to plant intruders there who will instigate violence.
The Joads settle into a comfortable existence at the government camp, and during the dance that Saturday, Tom and several other residents defuse the situation, preventing the police from taking control of the camp. Nevertheless, after a month in Weedpatch none of the Joads have found steady work; the family members realize that they must continue on their journey. They arrive at Hooper Ranch, where the entire family picks peaches. The wages they receive are higher than normal, for they are breaking a strike. Tom finds out that the leader of the labor force that is organizing the strike is Jim Casy. After spending time in prison, Casy realized that he must fight for collective action by the working class against the wealthy ruling class; Tom decides to join Casy in his efforts. Yet Tom, Casy, and the other strike leaders get into a fight with strike breakers, one of whom murders Casy with a pick. Tom struggles with the man and wrests away the weapon. He, in turn, kills the man who murdered Casy, and barely escapes capture by the police.
Although Tom wishes to leave the family in order to save them from taking responsibility for his actions, the Joads nevertheless decide to leave Hooper Ranch for a location where Tom can be safe. They reach cotton fields up north, where Tom hides in the woods while the family stays in a boxcar. Although the family attempts to keep Tom's identity and location a secret, young Ruthie Winfield reveals it during a fight with another child. When Ma tells Tom about this, he decides to leave the family and go off alone, determined to fight for the cause for which Casy died. He vows to return to his family one day.
The rainy season arrives almost immediately after Tom leaves the family; massive flooding results from this weather. The Joads are caught in a dangerous situation: they cannot escape the flooding because Rose of Sharon suddenly goes into labor. While other families evacuate the camp near the rapidly rising creek, the Joads remain and attempt to stop the flood waters. Without the aid of others, the Joads are unsuccessful, and they must seek refuge on the top of their car. Rose of Sharon delivers a stillborn child that Uncle John sends in a box down the creek. The family eventually reaches higher ground and finds a barn for shelter. Inside the barn is a starving man and his young son. Steinbeck ends the novel with Rose of Sharon, barely recovered from the delivery, breastfeeding the dying man to nurse him back to health.