Two men named George and Lennie are fleeing Weed, CA towards a ranch to seek a new job. They are running away, due to Lennie's wrong doing actions.
Lennie is mentally disabled, meaning that he has childlike behavior. This sometimes makes George scold him multiple times. George is also mad at him because Lennie is the reason why they had to flee Weed.
George and Lennie finally reach the farm. However, Curley dislikes Lennie, which creates a lot of tension on the ranch. Also, Curley's wife is constantly trying to seek attention from the men, which causes some problems with Lennie.
Curley's wife approaches Lennie and talks about various topics. She later allows him to touch her hair because Lennie likes to touch soft things. However, Lennie does not let go and she later panics. This causes Lennie to also panic, eventually killing her by breaking her neck.
Lennie realizes what he has done. He then flees to a bush near a river, because George told him to go there if anything bad happens.
Geroge has to kill Lennie because Curley's gang was approaching. If he did not kill Lennie himself, Curley may have killed Lennie in a much worse way.