For a very low-budget film that flopped when it was released in 1957, it's amazing that it is currently ranked 10th in the IMDB list of greatest films of all time. It's the top black-and-white film on the list, unless you count Schindler's List.
Apart from a very brief scene at the start and the end, the film is entirely set inside the jury room at a murder trial which requires a unanimous verdict and carries a compulsory death penalty, with the 12 men being the jury. In the jury room, there are no names given; and we only find out the occupations of a few of the men.
Obviously based on a play, lots of very clever lines, a good few twists, as each piece of evidence comes more and more under scrutiny.
The acting is excellent, there's good character development, it's very very good for thinking through how and why people think what they think or change their minds. Great film. Top 10 of all time? Maybe. But certainly the sort of film that people doing a job where the truth matters would do well to watch.