In the throes of the Great Depression, Tom Joad hitchhikes through Oklahoma after having recently been paroled from the local penitentiary. During his travels he meets Jim Casy. Tom remembers the man from his childhood, he was a preacher. However, as Jim explains to Tom, that while he is still a God-fearing man, he no longer feels a divine calling and has renounced priesthood. Tom offers for Jim to join him on his journey home. Jim accepts.
As the men arrive at the Joad farm, they are shocked to find it abandoned. A neighbour, Muley Graves, approaches the men and proclaims to Tom that the bank had been there earlier and had escorted the family off of the land. They had been staying with a relative and had plans to make the voyage to California in search of employment. Tom and Jim wait out the night at the now shuttered farm and venture to meet up with Tom’s family the next morning.
Tom arrives at the home of his Uncle and finds his family preparing to set off. Jim asks if it might be appropriate for him to join the Joad family on their trek, and they agree without hesitation. As soon as they have sold the last of their belongings and are ready to go – all except for a reluctant Grandpa who would much rather stay on his land than embark on a long journey West. Knowing that they are unable to leave their aging patriarch behind, the family drugs the old man and loads him into the truck.
As their journey starts out, the Joad’s come upon a young migrant couple – the Wilsons – whose car has broken down, but offer their tent to Grandpa who soon after succumbs to a stroke. Tom and Al help to repair the Wilson’s car and both families commit to finishing the remainder of the trip together.
As the families arrive in New Mexico, the car being driven by the Wilson family breaks down once more, forcing the group to stop. It is noted that Grandma’s health has taken a serious turn for the worst since the death of Grandpa, and Tom insists that the others should take the truck and go on without them. However, Ma refuses. She insists that the family must stay together. When they finally reach the California borderline, Sairy Wilson falls desperately ill and is unable to go any further. The families decide to separate and the Joads’ set off to cross the unforgiving California desert.
With the health of Grandma becoming increasingly concerning, Ma questions whether or not she will be able to withstand the trip across the desert. Yet, being unable to stop, Ma chooses to stay in the back of the truck with Grandma and consoles her as she silently passes away. As dawn arrives, the Joad family has reached the end of the desert and entered Bakersfield valley. Ma informs the rest of Grandma’s passing and insists that they give her a proper burial.
The family settles down for the night at the first camp they come upon, a shanty of tents and shelters and the men strike up a conversation with Floyd Knowles. The group is approached by a policeman and a businessman who offers them employment. Floyd requests a written wage offer and is accused by the policeman of being a ‘red’. As the policeman is trying to arrest Floyd, Tom trips him and Jim kicks him – rendering him unconscious. When the officer comes to, Jim turns himself in in order to divert attention from Tom who needs to continue on with his family.
As the Joad family continues to travel south, they find a government-run camp in Weedpatch, where they stay for just over a month but realize they must continue on.
Soon they are offered a job in Tulare, picking peaches. The gates of the camp are blocked by a large group of men waving and shouting. The family receives an escort from the state police and get to work. However, the payment of five cents per box of peaches picked is hardly enough to keep the family fed. After finishing his first day, Tom travels outside of the ranch and runs into Jim who is spearheading a strike against the farms owners – the workers are outraged at the orchard owners for wanting to pay just two and a half cents for every box of peaches picked.
Tom then realizes that he is only receiving five cents per box because he is working as a ‘scab’. While Tom and Jim talk, authorities sneak up and clobber Jim on the head, presuming that he is the leader of the strike. Jim is killed instantly. In a fit of rage, Tom begins to beat the assailant and ends up with a broken nose. Tom is able to flee, and hides in the orchard until he can safely reach his family.
Tom has become a fugitive, easily recognizable by his broken nose and scarred face. The family decides to leave the ranch as soon as the sun rises. They soon find employment picking cotton and take up residence in an empty boxcar with the Wainright family. Still fearing the authorities, Tom continues to hide and his mother leaves food for him. The Joad’s are comfortable and fed, however, soon Ruthie gets into an altercation with another child and threatens to contact her big brother, Tom, who has been hiding because he killed two men. This prompts Ma to instruct Tom to flee. Tom agrees and sets off to continue on the social good that Jim had started.
Eventually, Al Joad proposes to sixteen year old Agnes Wainwright. When the cotton season comes to an end, the rainy season begins. It rains long and hard, and the water levels rise steadily. Rose of Sharon goes into labor as the boxcar is on the verge of flooding. Pa, Uncle John, Al and the other men endeavor to contain the river by building an embankment, however, find their efforts pointless. The baby of Rose of Sharon is still born.
Several days later, the rain finally stops. Everyone, with the exception of Al and the Wainwright family, leave the boxcar in search of higher ground. The family ventures upon an old barn, currently inhabited by a young boy and his ailing father. The boy tells the family that his father is near starved to death and has not eaten in six days. His father, he says, struggles to keep down solid food. Rose of Sharon recognizes the horrible fate that awaits the man if he does not soon eat and offers him her breastmilk. The boy and the rest of the family exit the barn as she nourishes the dying man from her breast, while they lay on the dirty barn floor.