In Chapter One, Steinbeck introduces his two main characters, George Milton and Lennie Small. Lennie is mentally retarded, and George has to explain everything to him and tell him everything to do. Lennie usually obeys, although he sometimes becomes rebellious. The business with the mouse is introduced in this chapter. Along with showing their relationship, it foreshadows future trouble.
"What'd you take outta that pocket?"
"Ain't a thing in my pocket," Lennie said cleverly.
'I know there ain't. You got it in your hand. What you got in your hand--hidin' it?"
"I ain't got nothin', George. Honest."
"Come on, give it here."
Lennie held his closed hand away from George's direction. "It's on'y a mouse, George."
"A mouse? A live mouse?"
"Uh-uh. Jus' a dead mouse, George. I didn' kill it. Honest! I found it. I found it dead.
"Give it here!" said George.
"Aw, leave me have it, George."
"Give it here!"
George takes the mouse and throws it across the river, but when Lennie goes to collect firewood he retrieves and puts it back in his pocket. George sees Lennie's feet are wet from wading across the river and has to take the mouse away a second time.
Lennie likes to pet soft little things. From their subsequent conversation we learn that Lennie caused an uproar in the town of Weed by trying, according to his explanation, to fondle a girl's pretty dress. She thought he was trying to rape her and screamed. George and Lennie had to flee from a lynch mob and hide in an irrigation ditch until nightfall.
Lennie does not understand that his interest in soft things like mice and rabbits indicates an incipient sex drive. There are many references to the incident in Weed, but George only knows what Lennie told him because he came running when he heard the screaming; and Lennie shows that he is a compulsive liar. He told George he just wanted to feel a pretty dress, but he probably didn't realize he was attracted to the girl herself and was stupid enough to assault her in broad daylght on the main street. Since Lennie is attracted to little creatures, there is a good possibility that the "girl" in Weed was a minor.
This foreshadows the biggest event in the novel. Curley's wife foolishly invites Lennie to feel her soft hair when they are alone in the barn. He can't control his urges and won't let go. She begins screaming and he kills her while trying to shut her up. Curley's wife is very young herself, probably only sixteen.
George arrives on the scene after the girl is dead and Lennie has gone to hide at the riverbank campside where the novel opened. As George is looking down at the body, he realizes that Lennie is no longer a childlike creature who is only attracted to soft little animals, but has developed into a grown man with tremendous strength who is a potential rapist and killer. Curley's wife screamed because she thought Lennie was going to rape her--and she was right, although Lennie didn't understand his feelings any more than he understood them when he assaulted the girl in Weed. That incident, too, could have led to rape and/or murder.
George feels personally responsible for the death of Curley's pretty, young wife.
"I should of knew," George said hopelessly. "I guess maybe way back in my head I did."
George shoots his friend to save him from the lynch mob and also to expiate his own guilt for Lennie's crime. He could have helped Lennie escape across the river into the mountains, but Lennie would only have been executed for murder.
In his pocket Lennie has a mouse, a dead one. It says he loves to pet them but when they bite him he kills them. Mainly just to feel thier fur.