I have two impressions after watching this show.
1. It started as a mystery then evolved into a thriller and ended as something else.
2. It begged for a second season depending on how you interpreted it.

About the show.
The characters were all excellent. The female lead, Kurumi Nanba is played brilliantly by Ai Hashimoto. She is a strange genius, child like and innocent. She exhibits characteristics of Asperger's syndrome and functions well in her academic environment but seeks to have more. The male lead Tatsuhiko Handa magnificently portrays the unfolding of a detective who appears simple and humble at first glance who then exhibits a dark and mysterious side as he interacts with Nanba. The rest of the cast also give excellent performances.

The storyline begins as a simple mystery where the smart girl helps the silly cops. It soon gets much more complex as we see: that this innocent girl seems may have dark motives making one wonder if her appearance is a facade; hints that the simple and humble detective is really a much darker person from hints like his shady contacts and a questionable past; that there is more to what is going on than just simple cases perhaps a guiding evil.

The ending leaves us wondering. Wondering about Nanba and Handa's evolving relationship and characters. Wondering with such a menace emerging what will happen, wondering how deep goes the conspiracy, and much more. It begs for a second season to answer these question if you want concrete answers. If you want to imagine where it could go it has given a basis for many possibilities. I could see several different sequals based upon this beginning.