Act One, Scene 1: a clearing in the woods. George and Lennie are running from the police, and arrive in the clearing to hide. George vents his frustration at Lennie always getting the two of them in trouble, and George having to get them out (Trouble, always trouble: that's all you're good for.). This time, Lennie had been fascinated in a childlike way by the fabric of a woman's dress, but she'd interpreted his petting it as a rape attempt, and when she screamed Lennie had panicked and gripped tighter. George gripes about how much better his life would be without Lennie (My life would be so simple by myself). But he doesn't really mean it, any more than Lennie means his offer to leave (Just give me the word and I'll strike out alone). This seems to be their ritual.
Lennie has a mouse in his pocket; he had accidentally killed it by stroking it too hard. George, angry, throws it away, and Lennie laments (It was something I could stroke). George cheers him up by reciting their long-standing dream: to get a farm of their own, and live in security (One day soon, we'll save up enough). They lie down to sleep, haunted by the sound of police sirens in the distance.
Act One, Scene 2: the bunkhouse. Curley, the ranch boss, and Candy, and old crippled ranch-hand, are in the bunkhouse. Curley is cursing George and Lennie for arriving late (They said they'd be here this mornin', damn good-for-nothin' ranch-hands!). Curley's wife, young, bored, and frustrated, comes in to plead for attention and a night out (with Curley) (I want to go into town tonight). Curley throws her out churlishly, and she responds with equal churl. Both storm out.
In come George and Lennie, reporting for work. Candy sends them to their bunks, as the ranch hands return from the fields (Oh, I met her in Frisco in the month of July). Slim, the most respected of the hands, offers his dog's new litter of puppies to the men. Lennie, with childlike glee, asks George if he can have one.
The merriment is doused by the arrival of Curley's wife, who is ostensibly looking for Curley, but in fact is coming to flirt with the hands (He's always left when I've just come; he's always been where I'm just at). They know she is trouble, but she is also undeniably attractive. She discovers Lennie. But she leaves before anyone does anthing they regret. The hands hurl coarse jokes at her and Curley when she goes (Curley has married himself a tart!).
A hand named Carlson begins to tease Candy about how Candy's old and frail dog stinks up the bunkhouse. Slim raises the ante by suggesting that Candy shoot his dog and replace it with one of Slim's puppies (Your dog ain't no good to you no more). Carlson and the hands join in vicously (Get that ol' dog outa here!). Candy resists, but is browbeaten down. Carlson takes the dog and a pistol outside. For a long moment, tension rises in the bunkhouse as everyone waits for the shot. The voice of the Ballad Singer comes in from outside (Movin' on, always movin' on.), increasing the tension and guilt in the air. Finally, the shot rings out. The Ballad Singer confronts Slim. The Ballad Singer and hands sing of their longing for a home (One corner of earth to call my own). Night falls.
Act Two: the bunkhouse.As the hands play horseshoes outside, Slim and George relax in the bunkhouse. George is reading ads in the newspaper, looking for his and Lennie's dream farm. Slim is skeptical (Ev'ry ranch-hand I ever knew has had your dream of settling down). But George, furiously, insists that he and Lennie will find their dream (I'm not ready to settle for such a stingy life). And then, he spots an ad, and calls Lennie in to hear it (For sale cheap: five acre farm). Slim leaves, shaking his head.
Candy, who has been asleep on the bunk, asks if George is going to buy a farm. George resists the intrusion, but it turns out Candy has some money saved up, and offers to go in with them. After some figuring, George sees it will work (With what Candy's got an'with what we could save, I believe we just might swing her!). George, Lennie and Candy celebrate.
The celebration is doused, however, when Curley's wife arrives. George tries to throw her out, but just then Curley and Slim arrive. Curley is outraged to find his wife there. He tries to pick a fight with George, but then Lennie giggles, and Curley's rage is turned to Lennie. Curley beats on Lennie, who doesn't resist until the pain is too much and George lets him. Then all of a sudden he graps Curley's fist in his big hand and crushes it. Just as before, he panics and can't let go. George is heartsick: trouble with Curley means getting fired, which means no farm. But Slim blackmails Curley into not firing Lennie and George, by threatening to reveal how Curley hurt his hand. Searching for someone to blame, Curley settles on his wife. In anger, she says she is glad he got hurt. Curley beats her, and she flees in rage. Curley follows.
Candy tries to salvage the atmosphere by getting George to read about their farm again. The scene ends with the three of them, each in their own way, imagining the farm.
Act Three, Scene 1: the barn. Lennie is in the barn mourning his dead puppy (Why did you have to die?). Guess what, he had accidentally killed it by stroking it too hard. Curley's wife enters, looking for a discreet place to change into travelling clothes as she makes her escape from Curley and the ranch. Each sings of their dream: Lennie of a farm with lots of animals he can pet (Our farm's got a small house on five acres of land), and Curley's wife of the stardom that surely awaits her (Now I'm heading straight for Hollywood while I'm still young and still have my looks).
But she wants one last fling: she tempts Lennie down to stroke her soft hair. He loves it. In fact, he loves it so much that he won't stop. Curley's wife is wierded out, and screams. Lennie knows that screaming means trouble, which means they won't get their farm, so he puts his big hand over her mouth to silence her. Guess what, he accidentally kills her by pulling too hard. Aware that he did something wrong, he flees.
Candy comes in, looking for Lennie, and discovers the body. Shocked, he calls in George and Slim. Both understand immediately what has happened. Slim tells George to take a pistol and shoot Lennie himself, rather than let Curley and Carlson lynch him. Slim will delay the posse as much as he can. George sets off.
Act Three, Scene 2: a clearing in the woods. Lennie arrives, scared and cold (Oh, I feel cold inside.). He knows he's done something bad, but doesn't grasp the enormity of it. George catches up with him. Lennie wants George to rage at him like he always does, and George does so with a breaking heart (My life would be so simple... by myself). They hear the posse in the distance, and Lennie is scared (Are they lookin' for me, George?). George makes a mighty effort of self-control and reassures Lennie. He tells Lennie to look across the river and imagine their little farm. As George recites the dream one last time (One day soon we'll have us our farm), Lennie is finally able to see it (I see it, George!). And as Lennie delights in the vision, George pulls out the pistol and shoots him in the head. As George slumps to the ground, the posse arrives on the sad scene.