After the final closing arguments have been presented to the judge, she gives her instructions to the jury. In the United States (both then and now), the verdict in criminal cases (whether guilty or not guilty) must normally 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 teenage 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 (under current American criminal law, a defendant must first be found guilty or not guilty, and then the jury in the sentencing phase must find an aggravating circumstance to recommend the death penalty if the state has the death penalty). The jury of twelve retires to the jury room where they begin to become acquainted with each other's personalities and discuss the case.The plot of the film revolves around their difficulty in reaching a unanimous verdict, mainly due to several of the jurors' personal prejudices. 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 testimony promotes (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. After 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 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 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 maintained 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 to see 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", and 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.