I never thought that a K drama of past will make me so engaged and kept me guessing and theorizing this much after every episode. This drama not only has a solid plot but also intelligent characters even the villains are rational in their action.
Apart from the main plot and the relationship of its main group the story touches a lot of interesting points like - Importance of memory in a person's life, the role of memory in constructing ones identity, the idea of a crime free city and the importance of consent & privacy , treating foreigner as a threat etc
In every episode there were many thought provoking subjects difficult decisions to be made and there consequences to be faced in the future.
The way they connected the two time period at end was good the way they dealt with the shock was great too.
The only drawback I could see is a lack of heartfelt chemistry in the lead , I dont know if they purposely kept this way bcos the actors were great they just couldn't create a good setting for it.