Twelve jurors are sequestered to deliberate on the guilt or innocence of an 18-year-old boy accused of murdering his father. The jury initially seems to be unanimous in finding him guilty, but one juror (Juror 8, played by Henry Fonda) has doubts about the boy's guilt and sets out to convince the others.
The jurors engage in a heated debate, with various prejudices, biases, and personal experiences coming into play. Some jurors are quick to judge the boy based on his appearance, social class, and the prosecution's evidence, while others are more cautious and open-minded.
As Juror 8 methodically presents his arguments and challenges the others to consider the evidence more carefully, the jury begins to re-evaluate their positions. They examine the inconsistencies and weaknesses in the prosecution's case and gradually start to doubt the boy's guilt.
Throughout the deliberations, the jurors' personalities, motivations, and backgrounds are revealed. Some are driven by their own fears and insecurities, while others genuinely seek justice. As they confront their biases and engage in civil discussions, the jurors undergo a profound transformation in their understanding of the case and their fellow jurors.
In the end, the jury reaches a unanimous verdict of "not guilty," acknowledging that there is reasonable doubt about the boy's guilt. The movie emphasizes the importance of due process, careful deliberation, and the power of reason in reaching fair and just decisions.