The drama depicts a jury forced to consider a homicide trial. At the beginning, they have a nearly unanimous decision of guilty, with a single dissenter of not-guilty, who throughout the play sows a seed of reasonable doubt. The story begins after closing arguments have been presented in the homicide case, as the judge is giving his instructions to the jury. As in most American criminal cases, the twelve men must unanimously decide on a verdict of "guilty" or "not guilty". (In the justice systems of nearly all American states, failure to reach a unanimous verdict, a so-called "hung jury", results in a mistrial.) The case at hand pertains to whether a young man murdered his own father. The jury is further instructed that a guilty verdict will be accompanied by a mandatory death sentence. These twelve then move to the jury room, where they begin to become acquainted with the personalities of their peers. Throughout their deliberation, not a single juror calls another by his name because the names are unknown by the jurors. Several of the jurors have different reasons for discriminating against the witness: his race, his background, and the troubled relationship between one juror and his own son.
一名少年被控謀殺父親。法庭內，樓下老伯作供聲稱親耳聽到他大罵「我要殺死你」；對街女人作供聲稱親眼目睹他以刀刺進父親胸口；雜貨店老闆作供聲稱曾售賣兇器予少年。似乎，被判死刑將會是他的結局。
陪審團由十二位素未謀面的人組成，聽畢審訊退庭商議。無論陪審團作出任何裁決，必須一致。看似定局已成的判決由於其中一人的異議而爆發成激烈的爭論。十二名陪審員將如何決定少年的生死？
