At the close of a murder trial conducted in a New York City courtroom, the judge gives the jury its final instructions, reminding them that a guilty verdict will mean an automatic death sentence for the defendant, a Puerto Rican youth accused of killing his father. Once in the stiflingly hot jury room, Juror 3, a middle-aged businessman who is estranged from his own son, loudly proclaims that the boy is guilty and that all ghetto youths are criminals, while Juror 7, a fast-talking salesman, wants the jury to reach a decision quickly because he wishes to attend a baseball game that evening. Juror 1, the foreman, who is a genial high school football coach, conducts a preliminary ballot and, without hesitation, eleven jurors vote for conviction. Juror 8, a sensitive and thoughtful architect, casts the only dissenting vote, stating that he has doubts about the case and wishes to give the boy, who has had a difficult life in the ghetto, a fair hearing. Juror 10, approximately sixty years old and the owner of a garage, gruffly declares that the architect is a weak-willed "bleeding heart" before launching into a diatribe against slum dwellers. Wishing to restore calm, Juror 12, a young advertising executive, suggests that each juror present the reasons behind his verdict as a means of convincing Juror 8. The salesman, the garage owner and the businessman all suggest that the boy's ethnicity and class have been enough to convince them he murdered his father, while Juror 2, a shy and stammering bank clerk, appears to be maintaining his guilty verdict because he feels intimidated by the more outspoken jurors. Juror 4, a middle-aged and articulate stockbroker, and Juror 6, a young blue-collar worker, go over the evidence which determined their verdicts with much detail and thought. The prosecution has presented two seemingly reliable eyewitnesses, and motivation for the murder was suggested by the youth's frequent fights with his father. In addition, a shopkeeper identified the murder weapon as identical to an unusual and ornately carved knife he had sold the boy shortly before the murder. Finishing his exposition, Juror 4 offhandedly remarks that "everyone knows slums breed criminals," leading Juror 5, who until this point has remained silent, to declare with great dignity that he was raised in a slum. After Juror 8 points out inconsistencies in the prosecution's case and raises a number of questions, he throws down a cheap knife he bought near the courthouse which appears almost identical to the murder weapon. As many of the jurors begin to grow frustrated with the discussion, Juror 8 suggests that the foreman take a secret ballot from which he will abstain, promising that if all of them vote guilty this time, he will go along with them on the final ballot. Now, however, one juror out of the eleven votes "not guilty." Most of the jurors believe that Juror 5 has changed his mind, but the "not guilty" vote turns out to be that of Juror 9, an elderly and frail man to whom the jurors have, until now, paid little attention. After tempers have cooled down, Jurors 8 and 9 point out the inconsistencies in the prosecution's version of events on the night of the murder, and Juror 9 is especially convincing when he notes problems with the testimony of a prosecution witness who, like himself, is elderly. The two men manage to sway Jurors 5 and 11 to their side, for a total of four "not guilty" verdicts. Juror 10 now explodes with anger over what he views as "nitpicking" and Juror 3 harasses Juror 11, an Eastern European refugee, for changing his mind. After tempers subside, the weary jury continues its deliberations and when another ballot is taken, the tally is six to six, with Jurors 2 and 6 changing their original verdicts. Now at a complete standstill, some of the jurors want to declare a hung jury, but know that the judge will not accept the declaration without further deliberations. When Juror 11, who takes his duty as a citizen very seriously, questions whether all of the jurors have a clear understanding of "reasonable doubt," the obnoxious Juror 7 makes an angry speech full of anti-immigrant invective. Next, the newly confident Juror 2 asks how a 5'6" boy could have made a downward stab wound on a man who stood 6'2", leading Juror 5, who saw many a knife fight in the tough neighborhood in which he was raised, to convincingly demonstrate that the boy would most likely have held the knife underhanded, making a downward wound impossible. The foreman and Juror 12 eventually vote "not guilty," as does Juror 7, whose lack of concern over the case and desire to do whatever is most expedient greatly angers Juror 11, the immigrant. When Juror 8 asks the three remaining jurors to explain their continued insistence on a guilty verdict, Juror 10 makes an angry speech so full of hate and bigotry that everyone is shocked into silence. Juror 4, earlier so confident that the boy was guilty, admits he has reasonable doubt when the astute Juror 9 suddenly remembers that a female prosecution eyewitness had impressions on the sides of her nose of the sort left by eyeglasses. In support of their "not guilty" verdicts, the jurors realize that the witness deceived the court by taking off her glasses prior to her court appearance and they surmise that she was most likely not wearing them in bed the night she claimed to have witnessed the murder. Since Juror 10, who remains separated from the group because of shame over his outburst, has indicated he will change his vote, Juror 3 now stands alone in his conviction that the boy is guilty and he becomes increasingly belligerent and stubborn. When a picture of his son, who is only a few years older than the accused, unexpectedly falls out of his wallet, he suddenly breaks down into sobs and exclaims that all children are rotten ingrates. Overcome with emotion and guilt at the memory of his son, who rejected his harsh and authoritarian manner, he finally whispers "not guilty." As the jurors silently file out of the jury room, Juror 8 gently hands the distressed man his jacket. On the courthouse steps, Juror 8 and Juror 9 bid farewell, secure in the knowledge that they helped to ensure that personal prejudices did not determine the fate of the accused.