There is that quality of "What's going on?" which keeps one engrossed in this story. As one thing leads to another, we are challenged to make sense of what is happening and how all the people, places, and incidents are tied to each other. We know instinctively that the conclusions of the detective are wrong. Yet, it's a struggle to do him one better.
There are no "red herrings" and there is no deliberate attempt to lead one astray - so why is it so hard to figure it out? Of course it's because something is missing - but what? Well, trying to guess that "something" is fun. Also, watching a "detective" bumble his way through this case is fun - a lot of fun.
For the more technically oriented: Production values are high, acting is good, location shots are full of atmosphere, the music fits the tempo and is not jarring, the "feel" of the picture is that everything is real. I consider it dripping with authenticity.