First, Lennie has killed his puppy by smacking it because the puppy started to nibble at Lennie. Lennie is now upset because he thinks George won't let him tend the rabbits now.
You wasn't supposed to die. You wasn't that little. You wasn't supposed to die!
Curley's wife then walks into the barn to see what Lennie is up to. She then sees the dead puppy, but doesn't really care because all she wants to do is talk to Lennie. Lennie refuses to talk to her since George said not to.
Well, George won't know.
George said not to talk to you.
After that, Curley's wife begins to talk to Lennie about her life and how she could have been a movie star and how she doesn't love Curley. Lennie talks to her about their future life and he won't get to tend the rabbits no more. Lennie mentions that he likes soft things, which Curley's wife said that her hair is soft and that he could touch her hair.
Don't go messing it up now.
It's so soft!
Next, Curley's wife tells Lennie to let go of her hair, but Lennie doesn't, so she starts to scream. Lennie then grabs her by her mouth and neck telling her to be quiet otherwise George is going to be mad.
SHH! George is going to hear you.
AAAAAAHH!LET GO OF ME!
Then, Lennie kills Curley's wife accidentally because of his brute strength. Lennie is in shock looking at the dead body.
Finally, Lennie takes off running to the bushes that George told him to go to if he ever got into trouble. Lennie is repeating to himself, "I've done a bad thing."