The Grapes of Wrath takes place during the late 1930s and follows the journey of the Joad family as they travel from Oklahoma through Texas, New Mexico, and Arizona to California. The novel opens in Oklahoma, where farmers like the Joad family are facing severe conditions. The land is gradually eroding, leading to fierce dust storms. Tom Joad, the novel's protagonist, has just been released from prison. As he walks toward his house, he meets a former preacher named Jim Casy. Casy likes to talk about ideas, which amuses and annoys Tom.
Tom and Casy reach Tom's home only to find it abandoned. Tom learns that while he was away in prison, big landowners began driving tenant farmers from their land because of disappointing crop yields. Muley Graves, an old neighbor, tells Tom that the other members of the Joad family were pushed off their land and are now staying at Uncle John's place. Tom and Casy arrive at Uncle John's farm and greet the core members of the Joad family: Ma and Pa, Tom's parents; Noah and Al, Tom's brothers; and Grampa and Granma, Tom's grandparents.
Tom learns that many tenant farmers in Oklahoma are selling their belongings and traveling west to California, where there is rumored to be plenty of work. His family tells him that they are preparing to join this migration. Soon, several more family members arrive to greet Tom: Uncle John; Tom's younger sister, Ruthie; his younger brother, Winfield; his older sister, Rose of Sharon; and her husband, Connie. Rose of Sharon is pregnant. Casy asks if he can come with the Joads to California. The Joads welcome him, and they all load the truck. The Joads find it difficult to leave but feel that they must.
The Joads join numerous other migrants who are traveling down Highway 66 to California. At one point, the Joads stop their truck and meet a couple named Ivy and Sairy Wilson, who are repairing a car. Grampa is very sick and soon dies. The Wilsons help to bury him. The Joads and Wilsons decide to travel together to California. At an eatery on Highway 66, a waitress and cook show empathy for a migrant family, selling them food for a low price.
The Wilsons' car breaks down. Tom suggests a plan to fix the car, but it requires that the Joad family separate. Ma forcefully resists this plan, thereby taking a leadership role in the family. Tom implements a different plan to fix the car that keeps the family together.
The Joads and Wilsons arrive in California and are met with hostility from a policeman. Noah decides to stay by a river, leaving the family no choice but to go on without him. Sairy Wilson gets sick, causing the Wilsons to also stay behind. Ma worries about the family breaking up. Tom takes charge of the preparations for the trip across the desert. As the Joads cross the desert, Granma dies.
The Joads bury Granma. At this point, they realize they are among thousands of migrants flocking to California in search of work and housing, only to find that both are scarce. They soon arrive at a "Hooverville," a makeshift migrant camp, and are stunned by the squalor there. Not expecting such hardships, Connie abandons the Joad family, including his wife. It is now clear to the family that the number of migrant workers in California far exceeds the number of available jobs. Tom also realizes that the local landowners are glad for this, as it allows them to keep wages low and prices high. A contractor tries to hire workers from the camp without stating the pay. Angry about this, the workers, including Tom and Casey, attack the contractor. Casy takes the rap. Tom leaves the Hooverville with his family. As they depart, they see a mob raid the camp.
The Joads arrive at a government camp, where the migrants are allowed to govern themselves. Police are not allowed in the camp without a warrant. The Joads find the camp is well maintained. At a dance, however, men hired by the police start a fight with the migrants so that the police will have a reason to raid the camp. Tom and other migrant workers use nonviolent means keep the peace.
To keep prices high, the big landowners destroy some of their crops instead of letting hungry migrants eat them. This waste and cruelty causes a "crop" to develop in the souls of the migrants—the grapes of wrath. Ma tells the Joad men they have to leave the government camp to find work.
The Joads get work picking peaches at a ranch. A day's wage is barely enough to buy dinner. Tom sneaks out of the ranch and meets with people who are picketing the ranch. He finds that Casy is leading them. Casy tells Tom about the importance of all people working together to fight oppression. The police raid the protesters' camp, and Casy is killed. Tom kills the policeman who killed Casy and is injured himself. He sneaks back into the Joads' shack in the ranch. A posse starts to search for Tom. Ma decides that the family has to leave the ranch, hiding Tom in the back of the truck.
The Joads get work picking cotton, and Tom hides out in the willows to allow his injuries to heal. Ruthie tells some kids about Tom killing two men and hiding out. Ma brings food to Tom and tells him he has to leave. While hiding, Tom has realized the meaning of Casy's ideas. He realizes that people working together to fight oppression are stronger. Tom says good-bye to Ma. A rainstorm hits the cotton farm, including the nearby camp where the Joads are living. Rose of Sharon gives birth to a stillborn infant. The camp floods, forcing the Joads to evacuate. They go to a barn for shelter and meet a boy and his starving father. Rose of Sharon breastfeeds the starving man, thereby symbolically forming a community that extends beyond family. By helping each other in dire circumstances, people find a way to survive.
