In Henry Fonda's film career, this one is under the radar, and that's unfortunate, because it'a fantastic film that examines the system of law & order, and how it can screw up. There is a lot of grit and reality here, as often in real life, innocent people are convicted, and even executed. Often the state might have some reasonable suspicion that they may not be guilty, or may know their case isn't as strong as it should be, but they pursue and still get that verdict.
Eye witness testimony can often be a problem too... there have been cases where the science pointed in a different direction, but a jury went for an eye witness, and thus they were convicted. For example, how many people are in prison today for a rape they didn't commit? More than you might want to know.
That's what this movie makes you think about, as the system nearly leads them to their death. Henry Fonda may be a white man, but you could put any man or woman in this story and it still works, they can be black, or white, or any other race, and it still works.
Because this story reveals the truth, the system has flaws, the system is not perfect. Innocent people live out their lives in prison, or are executed. In 1939, the year this movie came out, there were 161 executions. How many of them were actually guilty? That question, is the very power of this film.