In the New York County Courthouse, a judge instructs a jury about to deliberate the case of an 18-year-old youth from a slum on trial for allegedly stabbing his father to death that if there is any reasonable doubt, the jurors are to return a verdict of not guilty. If found guilty, the defendant will receive a death sentence. The jury is further instructed that the verdict must be unanimous.
The evidence seems convincing, with testimony from the neighbor across the street who said she witnessed the defendant stab his father from her window. Further, the elderly neighbor in the apartment below testified that he heard the defendant shout at his father that he would kill him, and then, through his door’s peephole, saw him run past his door.
In a preliminary vote, all jurors vote “guilty” except Juror 8, who reasons that there should be some discussion about the case before quickly reaching a “guilty” verdict and also brings up the defendant’s rough upbringing. He questions the reliability of the two witnesses and the prosecution’s claim that the murder weapon, a switchblade, was “rare” and produces an identical knife. Juror 8 argues that he cannot vote “guilty” because reasonable doubt exists.
Having hung the jury, Juror 8 suggests a secret ballot among the other eleven; if all the remaining jurors still vote “guilty”, he will acquiesce. The ballot reveals one “not guilty” vote. Juror 3 accuses Juror 5, who grew up in a slum, of changing his vote out of sympathy, but Juror 9 reveals that he changed his vote, agreeing there should be some more discussion.
Juror 8 argues that the noise of a passing train would have obscured the threat that one witness claimed to have heard the defendant tell his father: “I’m going to kill you”. Juror 5 changes his vote, as does Juror 11, who believes the defendant, having returned to the apartment and been met by the police, was not trying to retrieve the murder weapon as it had already been cleaned of fingerprints. Juror 8 points out that people often say “I’m going to kill you” without literally meaning it.
Jurors 5, 6, and 8 question the witness’s ability to have made it to his door in time to see the defendant fleeing 15 seconds after hearing the father’s body hit the floor. Juror 3 is infuriated, and Juror 8 accuses him of being a sadistic public avenger. Juror 3 tries to attack Juror 8, shouting “I’ll kill him!”, and Juror 8 calmly replies, “You don’t really mean you’ll kill me, do you?” proving his previous point. Jurors 2 and 6 then change their votes, tying the verdict 6–6 as a thunderstorm begins.
Juror 4 doubts the defendant’s alibi, based on the boy’s inability to recall certain details, and Juror 8 tests Juror 4’s own memory. He is able to remember events from the previous seven days though with some difficulty. Juror 2 questions the likelihood that the defendant, being seven inches shorter than his father, could have inflicted the downward stab wound. Jurors 3 and 8 act this out which confirms the possibility. However, Juror 5 demonstrates the correct way to hold and use a switchblade, proving that someone skilled with a switchblade, as the boy would have been, would always stab underhand at an upwards angle against a taller opponent.
Impatient to leave due to having tickets to that night’s baseball game, Juror 7 changes his vote and earns the ire of other jurors, and is confronted by juror 11; he insists that he thinks the defendant is not guilty but doesn’t explain why. Jurors 1 and 12 then change their votes, leaving only Jurors 3, 4, and 10. Juror 10 erupts in vitriol against slum people. One by one, the rest of the jurors, except 7 and 4, turn their backs to him and Juror 4 tells him to “sit down and don’t open your mouth again”. Juror 8 reminds the rest that personal prejudice can cloud judgments. Juror 4 declares that the woman who saw the killing from across the street stands as solid evidence. Juror 12 reverts his vote, making the vote 8–4.
Juror 9, seeing Juror 4 rub his nose, irritated by his glasses, realizes that the witness had impressions on her nose, suggesting that she wears glasses but did not wear them in court. Other jurors confirm the same, and Juror 8 adds that she would not have worn them to bed, and the attack happened so swiftly that she would not have had time to put them on. Jurors 12, 10 and 4 then change their vote to “not guilty”, leaving only Juror 3.
Juror 3 gives an increasingly tortured string of arguments, building on earlier remarks about his strained relationship with his own son, the reason he wants the accused to be guilty. He tears up a photograph of him and his son and almost immediately breaks down sobbing and mutters “not guilty”, making the vote unanimous. As the rest leave, Juror 8 helps the distraught Juror 3 with his coat in a show of compassion. The defendant is found “not guilty” off-screen and the jurors leave the courthouse. In a brief epilogue on the courthouse steps, Jurors 8 and 9 introduce each other for the first time by their names (Davis and McCardle respectively) before parting.