Tom Joad, Jr., is released from the Oklahoma state penitentiary where he served a sentence for killing a man in self-defense. He travels homeward through a region made barren by drought and dust storms. On the way, he meets Jim Casy, a former preacher; the pair go together to the home of Tom’s family. They find the Joad place deserted. While Tom and Casy are wondering what happened, Muley Graves, a die-hard tenant farmer, ccomes by and discloses that all the families in the neighborhood have gone to California or are going. Tom’s folks, Muley says, went to a relative’s place to prepare for going west. Muley is the only sharecropper to stay behind. All over the southern Midwest states, farmers, no longer able to make a living because of land banks, weather, and machine farming, sold or were forced out of the farms they tenanted. Junk dealers and used-car salesmen profiteer on them. Thousands of families take to the roads leading to the promised land: California.
Tom and Casy find the Joads at Uncle John’s place, all busy with preparations for their trip to California. Assembled for the trip are Pa and Ma Joad; Noah, their developmentally disabled son; Al, the adolescent younger brother of Tom and Noah; Rose of Sharon, Tom’s sister, and her husband, Connie; the Joad children, Ruthie and Winfield; and Granma and Grampa Joad. Al has bought an ancient truck to take them West. The family asks Casy to go with them. The night before they start, they kill the pigs they have left and salt down the meat so that they will have food on the way.
Spurred by handbills that state that agricultural workers are badly needed in California, the Joads, along with thousands of others, make their tortuous way, in a worn-out vehicle, across the plains toward the mountains. Grampa dies of a stroke during their first overnight stop. Later, there is a long delay when the truck breaks down. Small businesspeople along the way treat the migrants as enemies, and, to add to their misery, returning migrants tell the Joads that there is no work to be had in California, that conditions are even worse than they are in Oklahoma. The dream of a bountiful West Coast, however, urges the Joads onward.
Close to the California line, where the group stops to bathe in a river, Noah, feeling he is a hindrance to the others, wanders away. It is there that the Joads first hear themselves addressed as Okies, another word for tramps. Granma dies during the night trip across the desert. After burying her, the group goes into a Hooverville, as the migrants’ camps are called. There they learn that work is all but impossible to find. A contractor comes to the camp to sign up men to pick fruit in another county. When the Okies ask to see his license, the contractor turns the leaders over to a police deputy who accompanied him to camp. Tom is involved in the fight that follows. He escapes, and Casy surrenders himself in Tom’s place. Connie, husband of the pregnant Rose of Sharon, suddenly disappears from the group. The family is breaking up in the face of its hardships. Ma Joad does everything in her power to keep the group together.
Fearing recrimination after the fight, the Joads leave Hooverville and go to a government camp maintained for transient agricultural workers. The camp has sanitary facilities, a local government made up of the transients themselves, and simple organized entertainment. During the Joads’ stay at the camp, the Okies successfully defeat an attempt of the local citizens to give the camp a bad name and thus to have it closed to the migrants. For the first time since they arrived in California, the Joads find themselves treated as human beings.
Circumstances eventually force them to leave the camp, however, for there is no work in the district. They drive to a large farm where work is being offered. There they find agitators attempting to keep the migrants from taking the work because of the unfair wages offered. The Joads, however, thinking only of food, are escorted by motorcycle police to the farm. The entire family picks peaches for five cents a box and earns in a day just enough money to buy food for one meal. Tom, remembering the pickets outside the camp, goes out at night to investigate. He finds Casy, who is the leader of the agitators. While Tom and Casy are talking, deputies, who have been searching for Casy, close in on them. The pair flee but are caught. Casy is killed. Tom receives a cut on his head, but not before he fells a deputy with an ax handle. The family conceals Tom in their shack. The rate for a box of peaches drops, meanwhile, to two-and-a-half cents. Tom’s danger and the futility of picking peaches drive the Joads on their way. They hide the injured Tom under the mattresses in the back of the truck, and then they tell the suspicious guard at the entrance to the farm that the extra man they had with them when they came was a hitchhiker who stayed behind to pick.
The family finds at last a migrant crowd encamped in abandoned boxcars along a stream. They join the camp and soon find temporary jobs picking cotton. Tom, meanwhile, hides in a culvert near the camp. Ruthie innocently discloses Tom’s presence to another little girl. Ma, realizing that Tom is no longer safe, sends him away. Tom promises to carry on Casy’s work in trying to improve the lot of the downtrodden everywhere.
The autumn rains begin. Soon the stream that runs beside the camp overflows and water enters the boxcars. Under these all but impossible conditions, Rose of Sharon gives birth to a dead baby. When the rising water makes their position no longer bearable, the family moves from the camp on foot. The rains had made their old truck useless. They come to a barn, which they share with a boy and his starving father. Rose of Sharon, bereft of her baby, nourishes the famished man with the milk from her breasts. The poor keep each other alive in the years of the Great Depression.