The entire play takes place in a shabby jury room on a hot day. The New York City skyline can be seen through the windows of the room. As the play begins, the room is empty, and a judge can be heard offstage giving instructions to the jury on how to deliberate on the case before them. The twelve men are to determine the fate of a 16-year-old boy accused of murdering his father. If they have reasonable doubt, they are to acquit the boy.  If the boy is convicted, he will face the death penalty.  

The jurors, who are known only by their juror numbers, file into the room. They discuss the heat that permeates the room and how it's the hottest day of the year. The 3rd Juror thinks that it's an open-and-shut case and is staunchly convinced of the boy's guilt. The 7th Juror is also impatient for the proceedings to wrap up because he has a ballgame that he's eager to attend. The men take their seats in order around the jury table and decide to cast their first vote on the verdict of the trial. Eleven of the men vote guilty and the 8th Juror is the lone standout. He believes that they should give the boy some of their time and discuss the ins and outs of the case. The jurors go around the room in order and discuss why they think the boy is guilty.  

Each juror’s deliberation reveals something integral about the case. The jurors discuss the testimony of the boy’s downstairs neighbor, an old man who claims to have heard the boy shout “I’m going to kill you” and had seen the boy run down the stairs after the murder. They discuss the boy’s alibi, which seems flimsy to them because he said he was at the movies when the murder took place but couldn’t recall the details of the film. They explore the testimony of the second witness, who claimed that she saw the boy murder his father through the windows of a passing train. The boy seems to have a clear motive, given that his father hit him earlier in the day, and he also has a history of violence.  

With each point that the other jurors make, the 8th Juror has questions, thoughts, or doubt. For example, he wonders if the old man could have identified the boy’s voice over the sound of the loud train, and he demonstrates that the old man likely couldn’t have gotten to the hallway quickly enough to see the boy pass by. This calls the old man’s testimony into question, as does the 9th Juror’s observations that the old man very badly needed the attention of the court, perhaps badly enough to skew his perspective. The 8th Juror interrogates the 4th Juror about the movie the 4th Juror saw earlier in the week, and, like the boy, the 4th Juror cannot remember crucial details of the movie, suggesting that the boy’s alibi could be valid. The 8th Juror points out that the boy had been hit by his father for his entire life and questions why the beating would suddenly drive the boy to murder.  

As the jurors discuss the case, personal biases begin to emerge. The 3rd Juror, a vocal advocate for a conviction, tells a story about his son, who he beat when he was young and who he is now estranged from. The 4th Juror coldly states that people from slums are more likely to be violent, which offends the 5th Juror, who grew up in a slum. The 10th Juror makes racist statements throughout, and as the deliberation goes on, it becomes clear he is less interested in following due process and more interested in seeing the boy punished because of his race.  

Throughout the discussion, more and more jurors change their votes to not guilty. There are three pivotal discussions that change the tide. First, the 8th Juror has the guard bring in the murder weapon, a unique knife. The prosecution argues at the trial that it was one-of-a-kind, but the 8th Juror produces an identical knife which he bought at a pawn shop close to the boy’s home. The 8th Juror makes the point that someone could’ve used a similar knife to kill the boy’s father. Second, the 9th Juror, noticing that the 4th Juror has indentations from his eyeglasses on either side of his nose, remembers similar marks on the second witness’s face. The 8th Juror points out that, if the woman was lying in bed, she wouldn’t have had her glasses on, making it unlikely she could’ve seen who killed the boy’s father. 

The final turning point comes when there is only a single man voting for a conviction, the opposite circumstance from the start of the play. The 3rd Juror angrily argues for the boy’s guilt, saying he can feel the knife going in his own body. The 8th Juror says that the boy on trial is not his son. He says that the 3rd Juror’s personal experience is clouding his judgement. The 3rd Juror relents and votes “not guilty.” The jurors leave the jury room, ready to return to the courtroom and acquit the boy.