The drama depicts a jury deliberating a verdict in a murder trial, in which the case at hand pertains to whether a young man murdered his own father. The story begins after closing arguments have been presented in the 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 jury is further instructed that a guilty verdict will be accompanied by a mandatory death sentence.
In the jury room, the first vote is a nearly unanimous decision of guilty, with a single "undecided" dissenter, who throughout the deliberations sows a seed of reasonable doubt. The jurors become acquainted with the personalities of their peers. Several of them have different reasons for discriminating against the defendant: his race, his background, and the troubled relationship between one juror and his own son. The one dissenter gradually wins over the other jurors to a unanimous not-guilty verdict, by questioning the reliability of the evidence presented in court and exposing his fellow jurors' prejudices.
The characters are unnamed; throughout their deliberation, not a single juror calls another by his name, and they are identified in the script merely by number.
