In the beginning of the drama, you find it very childish, like stories made for kids, 'that touch the animals butt and get know that they have seen.' In the first few episodes, I was not very fond of this drama. But when Dr. Bong and Detective Moon started arguing about stupid things, I felt it was interesting. It made me enjoy the drama. Not because I was curious about suspense, but because of the chemistry between both of them.
So in the very first place, the acting of the star cast was what engaged me to watch it further. I have watched so many series that, at some point, we can guess or suspect who is the mastermind behind the game before the series reveals it.  And my guess is always correct by 99.99% of the time. But I really could not tell until episode 15 was released. I used to think after every episode but couldn't reach the conclusion. I was really impressed by all the star cast and how they hide the serial killer very nicely through their acting.  It becomes interesting after Detective Moon's brother dies.
The story is not particularly good, but the director and cast make it worthwhile to watch.

I would like to give over all 8.5 stars out of 10 but 10 out of 10 for the acting of the entire cast.