I have been watchig kdramas for 10 years and I'm telling you this drama is on a scale of its own!
First of all the acting in this drama is beyond spectacular, the actors did a great job specially Lee Seung Gi. It's the first time he plays a role like this and he absolutely slayyy all his scenes I'm very proud of him.
The story line is completely different from the typical thriller drama, this one is full of and mystery (many people couldn't take it and drop it) too bad because is Masterpiece indeed.
The story line gets you involved since episode 1 and from there is a roller coaster, you start to suspect everyone to the point you just give up making theories and decide to just watch.
One thing I love and praise this drama is for giving us special episodes...that allow us to understand as it shows the different side of the coin.
Perfect ending writing is honest without any safe play they told the story with full of thrills from ep 1 to 20.