The young cop who still 'green' (referred so in the movie) takes responsibility for the missing gun and behaves in a way that is too sensitive for a cop. When he learns that some crook has got hold of his gun and had shot an innocent person while robbing he feels guilty and writes a resignation letter to the superintendent. Later the superintendent assigns him to investigate the case under a experienced cop.
The film essentially talks about the post war Japan. The crook whose is on the loose robbing people and killing them happens to be a war veteran who became desperate after his knapsack is stolen in the train while returning from the war field. This incident makes him so frustrated that he decides to take it out on the world.
Incidentally the cop who is in pursuit of his stolen gun and the mad dog killer happens also to be a war veteran and his knapsacks were also stolen when he was on the return journey. The screenplay is deftly crafted to include the intricate details of the homicide investigation team. The personal level in which the crime is conceived, approved and committed. In this respect it stays separate from the run of the mill kind of cop stories.
You can get more of the plot here. The interesting things I observed are the conversations in the movie. In most of the scenes there are three parties in the conversation. Framing of the shots is such that every conversation has unique appeal. The placing of the players in the shot is worth studying.
Watching masters of cinema in action restores my faith in genius over the equipments. A camera is a piece of machine without viewpoint. It is the visionary director who imposes viewpoint on the camera.
(Image courtesy: Wikipedia - http://en.wikipedia.org/wiki/File:Stray_Dog_1-30-27.jpg )