The play is set in a New York City Court of Law jury room in 1957. The play opens to the empty jury room, and the Judge’s voice is heard, giving a set of final instructions to the jurors. We learn that this is a murder case and that, if found guilty, the mandatory sentence for the accused is the death penalty. After these instructions, the jurors enter.

The men file in and decide to take a short break before deliberating. They complain that the room is hot and without air-conditioning; even the fan doesn’t work. All the jurors presume the obvious guilt of the defendant, whom we learn has been accused of killing his father. Eventually, the twelve sit down and a vote is taken. All of the jurors vote “guilty,” except for the 8th Juror, who votes “not guilty,” which, due to the requirement of a unanimous jury, forces them to discuss the case.
The jurors react violently against this dissenting vote. Ultimately, they decide to go around the table, explaining why they believe the boy to be guilty, in hopes of convincing 8th Juror.
Through this discussion we learn the following facts about the case: an old man living beneath the boy and his father testified that he heard upstairs a fight, the boy shouting, “I’m gonna kill you,” a body hitting the ground, and then he saw the boy running down the stairs. The boy claimed he had been at the movies while his father was murdered, but couldn’t remember the name of the movies or who was in them. A woman living across the street testified that she saw the boy kill his father through the windows of a passing elevated train. The boy had, that night, had an argument with his father, which resulted in the boy’s father hitting him twice. Finally, the boy has an extensive list of prior offenses, including trying to slash another teenager with a knife.
There is a strong rallying against the defendant. 3rd Juror compares him to his own son, with whom he was estranged, and 10th Juror reveals strong racist tendencies against the defendant.
When a discussion about the murder weapon, which was identified as the knife purchased by the defendant, a “one-of-a-kind” knife, begins, 8th Juror surprises the others by presenting an identical knife he had purchased in a pawn shop two blocks from where the boy lived a few nights prior, shattering the claim that the knife was so unique and identifiable.
8th Juror makes a proposition that the other eleven of them could vote, and if all of them voted “not guilty,” he would not stand alone and would go along with their guilty verdict. They agree to this and vote by secret ballot. The vote is 10 “guilty” votes and 1 “not guilty” vote, and so the deliberation continues.
Immediately, the jurors turn on 5th Juror, accusing him of having changed his vote out of sympathy for the boy. 9th Juror stands and admits to having changed his vote because he’d like to hear the arguments out.
8th Juror calls into question the validity of the testimony of the old man living downstairs. 9th Juror provides the possibility that the old man was only testifying to feel important. 8th Juror concludes by saying that even if he did hear him say, “I’m gonna kill you,” that very well could be taken out of context as just a figure of speech. With this 5th Juror changes his vote to “not guilty,” and the vote is 9-3 in favor of guilty.
After another heated discussion which raises the question of why the boy would have returned home, after killing his father, they take another vote. This time, 5th, 8th, 9th, and 11th vote “not guilty,” and the deliberation continues.
After a brief argument, 8th Juror brings into question whether or not the downstairs neighbor, an old man who had suffered a stroke and could only walk slowly, could have gotten to the door to see the boy run down the stairs in fifteen seconds, as he had testified. 8th Juror recreates the floor plan of the apartment, while 2nd Juror times him, and they conclude that he would not have been able to