I've described this film as surprising... this is true in many respects. The subject material (black man wrongly accused), the characters (people you expect to be stereotypes often show uncharacteristic attitudes during the film), the production...
All of these factors make for a refreshingly unusual film, especially for its time (1949). The only possible spoilers being the sometimes cheesy dialogue and occasional high moral stance.
But, if you happen across it when you weren't planning to watch a film, you might find yourself like me - staying up into the middle of the night just to see what happens.