Glengarry Glen Ross is the story of four Chicago salesmen—Levene, Roma, Moss, and Aaronow—and their supervisor, Williamson, who work together selling undesirable real estate at inflated prices. The play takes place at the end of a month in which the bosses of the company, Mitch and Murray, have declared a "sales contest": The salesman who clears a certain high dollar amount will win a Cadillac, and the two salesmen who perform worst will be fired. A chalkboard is used to keep track of each man's sales. Roma, who makes good sales, is the top man on the board, but the other three are all having trouble and getting increasingly worried.

The first act takes place at a Chinese restaurant near the office. In the first scene, Levene tries to convince Williamson to give him better sales leads so that he will have a better chance at closing a sale. Levene tries everything he can to persuade Williamson—bragging, threatening, begging, flattering, and finally bribing—but Williamson refuses to disobey company policy.

In the second scene, Moss and Aaronow discuss how unfair these policies are. Moss talks about a former salesman, Graff, who has gone off to start his own company. Moss tells Aaronow that Graff's business practices are both wiser and kinder than Mitch and Murray's. Moss suggests that someone should break into their office, steal the good leads, and sell them to Graff. Though Moss pretends at first to be harmlessly thinking out loud, it becomes clear that he really has planned this crime, and that furthermore he wants Aaronow to commit the break- in. If Aaronow does not rob the office, Moss threatens, then Moss will have to do it himself, and if he gets caught, he will tell the police that Aaronow was his accomplice.

In the third scene, Roma sits alone at a booth in the restaurant, and next to him, also alone, sits Lingk. Roma delivers a long and confusing monologue about the absence of an absolute morality in the world and the responsibility of each man to be the master of his own destiny. Lingk, though perhaps confused by Roma's ramblings, pays rapt attention. The scene ends just as Roma is about to begin a pitch to sell Lingk some land.

Act Two takes place the next day in the office. As per Moss's plan, someone has broken in the night before. A police detective, Baylen, is there interrogating the salesmen. Roma comes in from the street, having just heard about the robbery. He demands to know if the Lingk contract has been stolen—if it has been, then he will not win the Cadillac. Williamson assures Roma that the Lingk contract has been filed.

Levene comes into the office, jubilant because he has just closed a sale to an old couple, Bruce and Harriett Nyborg. He feels that after a long cold streak, his selling abilities have returned. Moss, coming out of the room where Baylen has just interrogated him, fumes at how the detective has treated him. Levene is still trying to tout the story of his tremendous sale, but Moss ignores him. This leads to a confrontation between Moss and Roma, in which Roma stands up for Levene and Moss accuses Roma of thinking he is the ruler of the office just because he happens to be on a hot sales streak.

Moss storms out and Levene finishes his story. Just then, Lingk comes to the office, having been sent by his wife to cancel the deal he had signed with Roma the night before. Knowing what Lingk has come for, Roma enlists Levene to pretend to be an important business client who needs to be rushed to the airport immediately. Roma pretends that he does not have time to talk to Lingk, but that he will get together with him to discuss the contract on Monday—when, as both Roma and Lingk know, it will be too late for Lingk to cancel the deal legally. Lingk does not want to disappoint Roma, but he is more afraid of disappointing his wife.

Roma promises Lingk that he has nothing to worry about, as the contract has not been filed yet. Roma is about to take Lingk out, to comfort him and try to save the deal, when Williamson, erroneously thinking that Lingk is upset by the disarray of the office, tries to help by reassuring him that his contract has indeed been filed last night. This directly contradicts what Roma has said, and Lingk realizes that he is being scammed. He leaves the office upset, planning to report Roma's sleazy practices to the Attorney General.

Roma is furious at Williamson for ruining his sale. He lets loose a stream of profanity at Williamson just before he goes into the inner office with Baylen to be interrogated. Levene picks up insulting Williamson where Roma left off. Levene accidentally lets on that he knows Williamson was lying about having filed the Lingk contract—and Williamson realizes that the only way Levene could have known that it had not been filed is if Levene was the one who committed the break-in.

Levene briefly attempts to deny the crime, but quickly folds and begins pleading with Williamson. He offers Williamson a percentage of his commissions. Williamson tells Levene that his commissions are worthless—the Nyborgs are notorious deadbeats, and Williamson knows that their check will not clear. Levene is humiliated by this revelation. Williamson goes to Baylen to report that Levene is the thief.

Roma comes out of his interrogation. Not knowing what has happened, he suggests to Levene that the two of them become partners. Levene is called into the inner office to be arrested by Baylen. When Levene is out of earshot, Roma tells Williamson that from now on, Roma will earn fifty percent of Levene's commissions, on top of the one hundred percent of his own. We see, then, that even Roma's kindness to Levene has been a scam all along.