12 jurors assemble in a New York County Courthouse to discuss the fate of an 18-year-old boy who has been charged with murdering his father. If he is found guilty, he will be sentenced to death. After an initial vote, the jurors all agree that he is guilty—all, that is, but juror 8, who disagrees, arguing that there is reasonable doubt that he did not.
8 makes a case for the fact that the two witnesses, an older man living downstairs from the family, and a woman across the street, are not necessarily reliable. He also finds an identical switchblade to the one used by the defendant, disproving the lawyers' claims that the weapon was rare.
When no one seems to budge, 8 recommends that they do another vote, and if everyone is still sure that the defendant is guilty, he will change his vote. When they vote again, Juror 9, an older gentleman, votes "not guilty," so they must discuss further. 8 makes an argument that a train was passing by when the downstairs neighbor claims to have heard the noise of the murder coming from upstairs.
They vote again, and Juror 5, who grew up in a slum himself, and Juror 11, who is a European immigrant, change their votes to "not guilty." 8 also points out that, since the weapon was cleaned of fingerprints already, it is unlikely that the defendant would have returned to his apartment later to retrieve the murder weapon, as the prosecution suggested.
They then discuss the fact that the downstairs neighbor cannot have possibly made it to the door of his apartment in time to see the murderer run down the stairs, as he alleged. Juror 3, who is the most intent on a "guilty" verdict, explodes at 8 and nearly attacks him, but is held back by the other jurors.
In another vote, 2 and 6 change their votes, and the voting is split down the middle. When Juror 4 doubts the defendant's alibi—the fact that he went to the movies, yet cannot remember what he saw—8 questions 4 about his own ability to recall details about the past.
When Juror 7 changes his vote just to keep the proceedings moving, Jurors 12 and 1 also change their votes, and Jurors 3, 4, and 10 are the only ones who still vote "guilty." 10 goes on a rant against people who live in slums and everyone leaves the table in protest of his bigotry.
Suddenly, Juror 9 points out that the witness across the street wears glasses, and is not likely to have been wearing them when she is said to have witnessed the murder. 10 and 4 vote "not guilty," seeing this as only adding to the reasonable doubt of the case.
Juror 3, the only one who still says that the boy is guilty, goes on an angry diatribe against his fellow jurors. In the midst of it, a photo of him with his son (from whom he is estranged) falls out of his pocket and he rips it up, bursting into tears. In the midst of his sobs, he mutters, "not guilty." The vote is unanimous.