John Steinbeck's novel The Grapes of Wrath tells the specific story of the Joad family in order to illustrate the hardship and oppression suffered by migrant laborers during the Great Depression. It is an explicitly political tract that champions collectivist action by the lower classes over expressions of individualist self-interest and chastises corporate and banking elites for shortsighted policies meant to maximize profit even while forcing farmers into destitution and even starvation.
The novel begins with the description of the conditions in Dust Bowl Oklahoma that ruined the crops and instigated massive foreclosures on farmland. No specific characters emerge initially, a technique that Steinbeck will return to several times in the book, juxtaposing descriptions of events in a larger social context with those more specific 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 at the diner, 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 with an interlude describing a turtle crossing the road, which he uses as a metaphor for the struggles of the working class.
On his travels home, Tom meets his former 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 women in his congregation. Casy espouses the view that what is holy in human nature comes not from a distant god, but from the people themselves. Steinbeck contrasts Tom's return with the arrival of bank representatives to evict the tenant farmers and the tractors to farm the land. He raises the possibility of a working class insurrection, but cannot find an effective target for collective action.
When Tom and Casy reach the Joad's house, it has been deserted. Muley Graves, a local elderly man who may not be sane, tells them that the Joads have been evicted, and now stay with Uncle John. Muley's own family has left to find work in California, but Muley decided to stay himself. That night, since they are trespassing on the property now owned by the bank, the three are forced to hide from the police who might arrest them.
Steinbeck follows this with a description of the tactics that car dealers use to exploit impoverished customers. They find that they can make a greater profit 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 prone to depression after the death of his wife several years before. His 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 and has the concerns befitting that age. This 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 based on flyers they found advertising work in the fields there. 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 people in the fields rather than preach at them. Before the family leaves, Grampa Joad refuses to go, but the family gives him medicine that knocks him unconscious and takes him with them. The subsequent chapters describes the vacant houses that remain after the Oklahoma farmers leave for work elsewhere, as well as the conditions on Route 66, the highway that stretches from Oklahoma to Bakersfield, California.
Almost immediately into the journey, the Joad family loses two members. The first victim is the family dog, which is run over during their 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 Wilson's 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 temporarily. Tom and Al do find the necessary part to fix the car at a junkyard, where the one-eyed man who watches over the junkyard complains about his boss and threatens to murder him. 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 the promises of work in the flyers are a fraud.
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 they stay, Granma becomes quite ill, but receives some comfort from proselytizing Jehovites who merely annoy Ma Joad. The police force them out of the camp, but the Wilsons choose the possibility of arrest instead, since Sairy Wilson is too sick to continue. The next time that the police stop the Joads on their travels, Ma Joad forces them to let them pass without inspection. She does this to hide from the police the fact that Granma has died.
Steinbeck follows this with a description of the history of California, which he frames as one 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 take what they need in order to survive.
At the next camp where the Joads stay on their search for work, they learn about Weedpatch, a government camp where the residents do not face harassment by police officers and have access to amenities including baths and toilets. When more police officers attempt to start a fight with Tom and several other migrant workers, Tom trips him and Casy knocks him unconscious. To prevent Tom from taking the blame, for he would be 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 in which the camp leaders are elected by the residents functions. Tom even finds work the next day, 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 and 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 his time in prison, Casy realized that he must fight for collective action by the working class against the wealthy ruling class. Tom, Casy and the other strike leaders get into a fight with strike breakers, and one of them murders Casy with a pick handle. 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 to spare them from taking responsibility for him, 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, and vows to return to his family one day.
The raining season arrived almost immediately after Tom left the family, causing massive flooding. 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.