George and Lennie are two migrant American labourers, who share a dream; that one day they may buy a farm, and Lennie will be able to take care of the rabbits.
Although Lennie is physically very strong and has the body of a man, he has the mind of a child.
The two men arrive on a ranch near the town of Soledad, where they are about to start work as barley buckers. On arriving there, they meet Candy, an old one-handed man who mops the floor, and Curley. Curley is the Boss' son, and immediately hates Lennie because of his impressive size. George realises that Curley will undoubtedly cause trouble for them.
After the two men are hired by the Boss, they meet Slim, a jerkline skinner. He has a lot of authority and has earned much respect from the men. They also meet Carlson.
Later that night, after dinner, Slim gives Lennie one of his puppies. Lennie is delighted by the gift, for he loves to pet things like rabbits, puppies and even mice, although he sometimes accidentally kills the creatures, not knowing his own strength.
George confides in Slim, telling him about how Lennie has caused many problems before, like in Weed. A girl thought Lennie was trying to rape her, and they had to flee.
Later that night, Candy overhears George and Lennie talking about their dream, and he asks if he could join them. He has quite a bit of money saved up, and they realise that if they combine their money in one month's time, they will have enough to buy the house. All three of them are excited by the prospect of how close they are to fulfilling their dream.
Still later that night, Curley attacks Lennie when he sees Lennie smiling when Curley is arguing with Carlson. Lennie does nothing to protect himself until George tells him to fight back. Then Lennie crushes Curley's hand. George is worried because he thinks he will be fired, but Slim convinces Curley to tell people that he got his hand caught in a machine.
The next night, (Saturday night) while most of the men are in town, Lennie visits Crooks, the Negro stable buck who lives alone in the barn. Crooks tries to explain the loneliness he feels but Lennie is too worried about George to listen.
After a while Candy also joins them, and when he talks about the house they are going to buy, Crooks becomes interested. However, when Curley's wife appears, looking for company because she is lonely, trouble starts.
When the three men refuse to answer her truthfully when she asks what happened to Curley's hand, she becomes angry. She guesses from the bruises on Lennie's face that he injured her husband in a fight.
Crooks becomes fed up with Curley's wife and asks her to leave. Curley's wife refuses, and insults Crooks, knowing that she can accuse him of molesting her, and she will be believed rather than the men.
Candy says he hears the men returning from town, and Curley's wife leaves, afraid that her husband might find her in the barn.
After Curley's wife has insulted, humiliated and degraded Crooks, he becomes disheartened and withdraws his offer to join Candy, Lennie and George on their farm. Candy and George both leave when George comes to find them.
The next afternoon, Lennie is in the barn. He is worried because he has accidentally killed his puppy and is afraid that George will not allow him to feed the rabbits if he finds out.
While Lennie debates whether or not to tell George, Curley's wife appears, and soon gets Lennie involved in a conversation with her, although George had warned Lennie never to talk to her.
She kindly lets Lennie stroke her hair, but when he won't let go she panics and struggles. She begins to scream, and Lennie is afraid he will get into trouble, so he shakes her to make her stop. He succeeds, but unfortunately he also kills her.
Remembering George's advice to run to the brush by the river if he is ever in trouble, Lennie does so.
Candy discovers Curley's wife, and fetches George. George has to tell the other men, although there will be little chance of taking Lennie alive if Curley finds out that he killed his wife.
When the men hear the news they go into a sort of killing frenzy. When Carlson finds his gun missing, he reports this to Curley, and it is decided that there can be no mercy for Lennie since everyone assumes he has the gun. Without further ado, the manhunt for Lennie begins.
Lennie waits for George to come and fetch him, and while he does, he imagines both his Aunt Clara and a giant rabbit reproaching him. Then George appears.
Lennie persuades George to tell him his favourite story about how he will feed the rabbits. As George tells the story, he withdraws Carlson's gun from his pocket, and at the end of the story, he mercifully kills Lennie by shooting him.
When the other men arrive they are amazed that George managed to get the gun from Lennie and then shoot him with it, except for Slim, who immediately understands what really happened and offers George consolation. The two friends walk off together in sorrow, mourning the loss of their mutual friend.