The land is changing.
There's been a drought, dust storms are rampant, crops are dead, the economy is weak, and landowners have to kick tenant farmers (like the Joads) off of the land.
The Joads must move westward.
The Joads must leave their home and their land, and seek a new life in California. With no work here in Oklahoma, they have no other choice.
California ain't paradise.
Over three hundred thousand people have moved to California, all looking for the same thing: jobs. The Joads can't find steady work and can't settle down.
Tom kills a man.
Tom is reunited with Reverend Casy, who has become an "agitator" or a picketer, fighting for worker rights. The authorities murder Casy, and Tom kills one of the authorities. He's in big trouble now.
Ruthie spills the beans. The floodwaters rise.
Tom tries to hide from the world until the murder he's committed blows over, but his little sister accidentally spills the beans and tells the world what he's done. Meanwhile, it starts raining really hard and the creek beside the Joads' boxcar home rises.
Tom runs away, the Joads escape.
Not wanting to drag his family into his crime, Tom runs away for good. As the floodwaters continue to rise, Rose of Sharon's baby is stillborn. The Joads leave their boxcar in search of higher and dryer ground.
A dying man is nursed.
In a barn, Rose of Sharon nurses a half-starved man with her own breast milk.