The cotton pickers are paid by the pound. The landowner has his own scales to weigh the cotton, which means that the scales are not always accurate. The cotton pickers try to increase the bag weight by adding rocks to their bags, so either way someone is always trying to take advantage of someone else. The pickers also have to pay for their own bags, which cost a dollar. If the picker does not have a dollar then the money is taken out of their pay. This can cause a problem because over a hundred people are working in the field at the same time, which means it is quite possible to make less than a dollar from picking cotton. The picker would then walk away from a day's work with no pay at all.

The Joads make a home for themselves in one of the boxcars that are lined up by the stream. Tom is hiding out in the brush so Ma can bring him food every night. They share their boxcar with the Wainwright family. They are bringing in money by picking cotton, so they can afford some new clothes and good food.

One day, while sitting outside the store, Ruthie has a fight with a girl over Ruthie's box of Cracker Jacks. While trying to scare the girl, Ruthie tells her about Tom. She tells the girl Tom has killed two men and is hiding out. Winfield tells his mother what Ruthie has said, because he wants to get his sister in trouble. Instead, Ma takes food and money out to Tom and warns him that trouble might be coming. Neither one of them are mad at Ruthie, because the girl is feeling so bad already about what she has said. Tom and Ma decide it would be best if Tom left. Ma makes him take the seven dollars she has been saving, so he will have some money for his journey. Tom has been thinking he would like to be an organizer like Casy, because he wants to help the migrant farmers. He promises he will find his family after it is safe for him to come home.

On her way back home, Ma is approached by a man looking for cotton pickers. Ma tells him her family would pick cotton for him. Back in the boxcar, she sees Mr. Wainwright talking to Pa, he is concerned about Al and Aggie's relationship. Aggie is Mr. Wainwright's daughter, who is sixteen years old, he is afraid the two will bring shame on his family if Aggie becomes pregnant before being married. Ma promises she and Pa will have a talk with Al. When Al returns to the boxcar he announces his engagement to Aggie. Both families are very happy about the news, but Ma has told Al he cannot leave the family until spring. They need Al to drive the truck to the various farms where they work.

The next day, Rosasharn decides she is going to help the family pick cotton even though she is close to having her baby, because her husband has not returned from the walk he took and she feels she should make some money. She becomes very ill from the work.

The rains come to California. It rains until the land is soaked, it rains until the land can't hold anymore water and the floods come. The people cannot work, so they start to beg for food and when that fails they start to steal. The sheriff deputizes more men and orders more guns. The migrant workers start to die from disease and starvation and the local people become angry and afraid of the migrant workers. Even after the rain finally stops, the workers know they will have to wait three months until the fields will be ready to pick again.

The rains come to the boxcars and fill the stream which the boxcars sit by. The water is rising and the people are afraid of being flooded out. Pa has the idea that if the men of the camp work together they can build a mud wall to keep the water from flooding the camp. Some of the men need to be convinced that this could work. The Wainwrights want to just leave, which means Al would leave with them, but Pa talks them into building the mud wall.

Rosasharn is sick with chills and a fever, she has no appetite and Ma is very worried about her. Rosasharn goes into labor early, possibly because she is so ill. The men labored to build the wall as she labored to give birth to her child. The wall is demolished by a tree which has fallen into the stream and the baby is stillborn. The Joads build a platform to put their possessions on because the water is now coming into the boxcar. Uncle John is given the task of burying the baby, instead he sets it adrift in a box down the stream. He wants it to be found to show the people what is happening to the migrant families. Eventually Ma has had enough of being wet and decides it is time for everyone except Al to move on. Al is staying behind to guard the truck and their possessions. The family trudges through the water on the state road until Ma spots a barn. She thinks the barn should be dry, so they make their way to the barn. In the barn is a boy and his dying father; the man hasn't had food for six days. Rosasharn saves his life by letting the man nurse from her.

The family endures the pain of losing Rosasharn's baby, Tom having to leave the family, and being flooded out of their boxcar. But in the end Rosasharn gives hope to a dying man by saving his life with the milk meant for her child.