In the exposition of Of Mice and Men, George Milton and Lennie Small are preparing for their next job on a ranch near Soledad. The two have been traveling for a while after leaving the previous ranch and decide to spend one final night relaxing on the bank of the Salinas River. It is clear from these earliest lines that Lennie has cognitive challenges, and George looks out for him. Lennie tries to hide a dead mouse from George; he explains that he just wants to stroke something soft as he walks along. George dreams about the future, imagining the house they will one day be able to afford; Lennie is excited at the idea of having rabbits on their future land because he wants to care for them. George instructs Lennie to return to this same spot by the river if he should ever get into trouble on the ranch.

The rising action begins when George and Lennie arrive at the ranch. They meet various characters who also work there, such as Candy, an old swamper who is missing a hand. Candy warns George and Lennie to watch out for Curley's wife, alluding to her tendency to stir up trouble for men; Curley is the boss's son and is a quarrelsome man. Curley's wife soon appears at the bunkhouse, dressed in clothing that is inappropriate for her setting. George defers her questions, despite her "playful" attempts to engage him. After she leaves, Lennie insists that she is "purty."

Lennie is excited to find out that Slim, another rancher, has a dog who is about to have puppies; Slim promises Lennie that he can have one when the puppies are born. Curley drops by the bunkhouse looking for his wife and becomes convinced that she is sneaking around with Slim. Later that night when Candy brings his old dog inside, Carlson is disgusted by the dog's smell. He insists that Candy needs to shoot the dog and then offers to do it himself. Although Candy doesn't want to comply, he eventually relents; the group of men listens as Carlson leads the dog away and then fires a gun.

Curley returns to the bunkhouse and begins antagonizing Lennie. At first, Lennie tries to retreat from Curley's blows, refusing to even put his hands up to defend himself. Finally George encourages Lennie to "get him," so Lennie grabs Curley's hand and squeezes it until Curley is "white and shrunken." Curley begins crying, unable to free himself from Lennie's grip. Satisfied with this resolution, George orders Lennie to release the hand, and Curley leaves to seek medical treatment, promising not to tell anyone what happened.

When the puppies are born, Lennie is initially excited; unfortunately, he accidentally kills his puppy and wants to hide the truth from George. Curley's wife enters the barn where Lennie mourns the loss of his puppy, and she is uncharacteristically warm and engaging. The climax of the story occurs when Lennie tells her that he loves petting soft things, and she offers to let Lennie stroke her hair. When she asks Lennie to stop, he panics and accidentally breaks her neck. He then hides the body and runs away.

The falling action begins when Candy finds the body and reveals the tragedy to George. The other men quickly decide that they must find Lennie and kill him, ignoring George's pleas to spare Lennie's life. George sneaks away from the group, going to the banks of the Salinas River, where he had told Lennie to hide if he ever got into trouble. Indeed, Lennie is waiting there for George and begins apologizing for what has happened.

The resolution occurs when George instructs Lennie to look out across the river and to imagine their future farm once again. Lennie is filled with joy, reminding George that they will "live on the fatta the lan'." George places a pistol at the back of Lennie's head and pulls the trigger. Lennie dies just before the other men appear.