When the final closing arguments in a murder trial have been presented to the judge, she gives her instructions to the jury, all of whom are men. In the United States, the verdict in criminal cases must be unanimous. A non-unanimous verdict results in a hung jury which in turn forces a mistrial. The question they are deciding is whether the defendant, a teenaged boy from a city slum, murdered his father. The jury is further instructed that a guilty verdict will be accompanied by a mandatory death sentence. The jury of twelve retires to the jury room, where they begin to become acquainted with their personalities and discuss the case.
The story revolves around their difficulty in reaching a unanimous verdict, mainly because of the personal prejudices of several of the jurors. An initial vote is taken and eleven of the jurors vote for conviction. Juror number 8, the lone dissenter, states that the evidence presented is circumstantial and the boy deserves a fair deliberation, upon which he questions the accuracy and reliability of the only two witnesses to the murder, the fact that the knife used in the murder is not as unusual as the testimony indicates (he produces an identical one from his pocket), and the overall shady circumstances.
Having argued several points, Juror 8 requests another vote, this time by secret ballot. He proposed that he would abstain from voting, and if the other eleven jurors voted guilty unanimously, then he would acquiesce to their decision. However, if at least one juror voted "not guilty", then they would continue deliberating. In a secret ballot, Juror 9 is the first to support Juror 8, and not necessarily believing the accused is not guilty, but feeling that Juror 8's points deserve further discussion.
After hearing further deliberations concerning whether one witness actually heard the murder take place, Juror 5 (who grew up in a slum) changes his vote to "not guilty." This earns criticism from Juror 3, who accuses him of switching only because he had sympathy for slum children. Soon afterward, Juror 11, questioning whether the defendant would have reasonably fled the scene and come back three hours later to retrieve his knife, also changes his vote. Jurors 2 and 6 also decide to vote "not guilty" to tie the vote at 6-6, Juror 7 (who has tickets to a baseball game at 8:00 that night) becomes tired and also changes his vote just so that the deliberation may end, which earns him nothing but shame. When pressed by Juror 11, however, Juror 7 says he believes the defendant is not guilty.
The next people to change their votes are Jurors 12 and 1 when Juror 8 demonstrates that it is unlikely that one witness actually saw the boy flee the scene, making the vote 9-3. The only dissenters left are Jurors 3, 4, and 10. The remaining jurors are intrigued when Juror 11 proves that although the psychiatric test presented in the case stated that the boy had subconscious desires to kill, tests of such do not prove anything other than what could possibly happen. Outraged at how the proceedings have gone, Juror 10 proceeds to go onto a bigoted and narrow-minded rage on why people from the slums can't be trusted, and as he speaks, Juror 4 responds: "Sit down. And don't open your filthy mouth again."
When Juror 4 is pressed as to why he still maintains his vote, he states his belief that despite all the other evidence that has been called into question, the fact remains that the woman who saw the murder from across the street still stands as solid evidence. After he points this out, Juror 12 changes his vote back to "guilty" to make the vote 8-4 again. Then Juror 9, after seeing Juror 4 rub his nose (which was being irritated by his glasses), realizes that, like Juror 4, the witness who alleged she saw the murder, had impressions in the sides of her nose, indicating that she wore glasses, and likely was not wearing them when she saw the murder. After he points this out, Jurors 12, 10, and 4 all change their vote to "not guilty".
Last of all to agree is the rigid Juror 3 who is forced to present his arguments again. He goes off on a tirade, presenting the evidence in haphazard fashion, before coming to what has really been bothering him all along: the idea that a son would kill his own father (it was established earlier in the film that Juror 3 had a bad relationship with his son). He begins to weep and says he can feel the knife being plunged into his chest. Juror 8 points out quietly that the boy is not his son, and Juror 4 pats his arm and says: "Let him live." The man gives in. The final vote is unanimous for acquittal.
All jurors leave and the defendant is found not-guilty off-screen, while Juror 8 helps the distraught Juror 3 with his coat in a show of compassion. In an epilogue, the friendly Jurors 8 (Davis) and 9 (McCardle) exchange names (all jurors having remained nameless throughout the movie) and part ways as Juror 3 walks slowly alone.The Foreman/Juror #1 (Courtney B. Vance): A high school football coach; he tries to keep order in the hostile jury room.
Juror #2 (Ossie Davis): A meek bank teller who does not know what to make of the case.
Juror #3 (George C. Scott): A businessman with a hot temper. He has a strained relationship with his son. He is convinced that the defendant is guilty, though it may not be through the facts of the case.
Juror #4 (Armin Mueller-Stahl): A stockbroker; he is very eloquent and looks at the case more coherently than the other jurors: through facts and not bias. He is appalled at some of the behavior of the other jurors (especially Jurors 3, 7, and 10.)
Juror #5 (Dorian Harewood): A health care worker (possibly an EMT); he is from the Harlem slums; he connects with the man at trial and is disgusted at the bigotry of Juror 10.
Juror #6 (James Gandolfini): A house painter; he is patient and respectful of what other people have to say.
Juror #7 (Tony Danza): A salesman; he is not concerned at all about the young man on trial, and eager to complete their jury assignment so he can use his tickets to a baseball game that evening. He is impatient and rude, and likes to crack jokes a lot.
Juror #8 (Jack Lemmon): An architect who is very quiet, and has two children. He is the only one of the twelve who, at first, votes not guilty. He becomes close friends with Juror 9 at the end of the film. His real name is Davis.
Juror #9 (Hume Cronyn): A wise old man who sides with Juror 8 and becomes friends with him at the end of the film. His real name is McCardle.
Juror #10 (Mykelti Williamson): A carwash owner; Former member of the Nation of Islam, he is a loudmouth, narrow-minded bigot, extremely rude and often interrupts people, who feels that no good thing will come out of the boy's "kind". Ultimately he is shunned from the group by the eleven men, with Juror 4 ordering him to "sit down" and to "not open his filthy mouth again."
Juror #11 (Edward James Olmos): An immigrant watchmaker, he believes in justice in America and will see it get done. He is observant of events around him.
Juror #12 (William Petersen): An ad executive; he is swayed very quickly by others' opinions, and does not have a full understanding of the life at stake outside of the jury room.
Mary McDonnell – Judge Cynthia Nance
Tyrees Allen – The Guard
Douglas Spain – The Accused