This drama kind of caught me off guard: I started watching it with a prejudice against idols acting. I usually think they don't bring enough depth to the characters, that they try too hard and makes it difficult to take them seriously - idols are not actors after all. However Park Jin Young and Shin Ye Eun quickly turned out to be fascinating through the raw emotions they conveyed: they don't try too hard nor not enough, they are spontaneous and hit the emotions just right.
I have to say, they are backed by two actors who did more than good; Kim Kwon and Kim Da Som perfectly embraced their roles and they grasped the personalities of their characters in a way it makes it easy to jump right into the plot.
So after praising the actors, I should praise the plot right?
Well that's where the drama loses points. Although it was a nicely built story, I was still bothered by some incoherences and the inevitable moments of boredom. The plot revealed too much, too quickly: the multiple flashbacks sure were interesting and nice, but I do believe they weren't exploited to the maximum of their potential. Which leads me to the main point of my critique: suspense fell flat almost all through the drama for me. If you paid enough attention to what was going on, you knew what would happen next, and in my opinion, the writers didn't succeed in building up anticipation and dread, which are vital for that kind of thriller drama.
To sum up, the drama is worth watching, for the original plot as well the skilled actors, but could have been much better plot-wise.