A good detective story accurately portrays the crime and the logical analysis which eventually reveals it. A "great" detective story is something which goes beyond it.
First, it focuses on the social aspects of the story by describing the characters and the often complex relationships between them. Second, it not only reveals the criminal but also reveals why might he have committed the crime or for that matter what situations might have led (forced) him to commit the crime. Towards the end of a great detective story the reader (viewer) is bound to empathize with the criminal.
This film is a great cinematic version of a great detective story. The acting is excellent. Abir's role as the detective Byomkesh playing battle of wits on one hand, and becoming a dutiful husband back in his home on the other would have made the author (Sharadindu) proud. Shaswata plays a cameo in a short role as Ajit. Generally speaking, the direction and screenplay are good, and cinematography is excellent accurately capturing the period of the early 70s.
The film is not flawless though. The character of Benimadhav is not very well developed. Moreover, what Byomkesh refers to as a Bengali novel was actually a book of poems by Tagore.
Besides these little flaws, the film is almost perfect and is definitely the best Byomkesh film delivered till date.