I am not one to use the phrase "kept me on the edge of my seat" since that's not my style, but this one sure did keep me on the edge of my seat, and I am not complaining.

I will start from the plot: it made me doubt most of the characters' motives, even when they seemed "obvious". The writer successfully weaved the story so that at one point or another, the main characters' stories coincided. The progress of the story is satisfying, with well-timed revelations that moved on to a stronger build up without losing the viewers' interest - well, at least mine. The one aspect that I loved was the tangible tension between the characters; they know who the other is, but they face each other, talk among themselves and drop subtle hints for the one harbouring suspicion. Many felt unsatisfied by the last episodes, including me, but leaving things open-ended is the right way. The characters deserved closure, after all they went through.

The characters had many sides to them. No back story is meant to be an apologetic defense for their crimes, and this is what I call being grey. The writer(s) was not trying too hard, which is admirable. You are made to understand how a range of circumstances out of your control can influence your decisions, the good and the ugly. The questions Lee Joo-Young asked his psychologist made you ponder - exactly what is so wrong about hurting others? I found myself sympathizing with the serial killers: in their own twisted way, were they not restoring justice that many were deprived of? You cannot truly despise someone here, but it did not steal my want for justice either. The one thing I find a little irksome is the writer's decision to dumb down other characters to show Lee Hyun's genius - they did not have to do that. But then, it is easier to see some things in third person perspective sometimes. Cha Ji-An especially struck me as written off a little clueless, but later I understood she really is smart - she understood Lee Hyun's dread without his voicing it; she put clues together with small hints; she was emotionally aware, something I admire in people.

The acting was excellent. Park Bo-Gum, Do Kyungsoo (although only a cameo role) and Choi Won-Young enraptured me with their performance. Of course, Jang Nara and Seo Inguk are worth mentioning, but the roles the first three had to play were, in my opinion, a more complex. These actors embodied their roles perfectly: Kyungsoo's line delivery, Bo-gum's slight shifts in empressions, and Won-Young's performance that confused most of us (in a good way). Seo Inguk was the perfect fit for a frustrating genius while Jang Na-ra showed us a determined, strong woman - not the ideal strong, but the relatable one. I honestly think these actors, including the supporting ones, carried the drama. One has to applaud.

The brotherhood was one of the major the driving forces of this drama. It was so bittersweet and well done. Many have elaborated on that so I won't do so. I will move to the unpopular relationship (judging by the comments): I loved the ML and the FL together. Many felt Cha Ji-An was an unnecessary character, but, honestly, had it not been for her, Lee Hyun would not have the stability he needed to come to terms to his identity. Jung Sun-Ho was too confused to have been Lee Hyun's rock. Which leads me to another point: the romantic relationship was not overdone, and was based upon mutual understanding and strength more so than chemistry that k-dramas usually rely upon; the trust and love towards each other was gradual, believable, and sweet. The small moments were endearing and made me root for the two until the end. Much of the character development Lee Hyun went through was by understanding Cha Ji-An's struggles, along with other realisations I will not mention.

The music was beautiful, if we talk about the OSTs. The first one was a tear-jerker (they always placed it on the right time), and the second one was befitting for the ML and FL. But sometimes, during the softer moments, the music switched to a more upbeat quality, stealing the vibe of the overall episode. A gentler tone would have worked better.

The rewatch value is strong in this one. I know all the twists and turns, but the beauty of the relationships and the outstanding performance of the actors make me miss the drama. I am sure I will watch it again in a few weeks.