The play, based on a film that was adapted from Reginald Rose's teleplay at the time, begins with an eighteen-year-old boy from a slum who is on trial for the murder of his abusive father. A jury of twelve people is locked in the deliberation room to decide the fate of the young boy. All evidence is against the boy and a guilty verdict would send him to die in the electric chair. But the judgement must be unanimous. Even before they begin a discussion, it is apparent that most of the jurors are certain the boy is guilty.

While most jurors have already decided on the fate of the boy, one vote of not guilty opens Pandora's Box and a forced deliberation begins. Slowly, but surely, each member of the jury is forced to confront the facts on hand and open his/her mind to the possibilities that exist outside of them, and shine a light on the kind of person he/she actually is.