Firstly, this is not a show you can binge watch.   Each episode is over an hour long.  The pacing of the story is slow and draggy.  They filmed each episode like a movie which is great to look at but not good for binge watching. I actually felt exhausted after each episode partly due to the emotional heaviness of the content and the pacing of the show.  

Pros:  I can see why this drama is rated so high. The cinematography and directing of this drama is like a movie.   The story was very well plotted and executed.   The plot itself was not overly complex, (detective in the present time converses with another detective in the past and together they help each other solve Korea's notorious cold cases).  The intrigue really comes from how the director chose to tell the story.  Pieces of the plot is told in fragments leaving the viewer wondering, how in the world does this information relate to that information.  Only during the last episode do you start to see how EVERYTHING is tied together.   The main characters were all well cast and played their parts well.  Shout out to the actor that played Lee Jaehan.  

Cons:  The reason why I couldn't give this drama a higher rating was because I felt like the cons of the show over shadowed the pros.  One major con was the pacing of the show.  The show dragged...  It was not even draggy like how Chinese dramas tend to fill up the dead time with stupid side arcs that have nothing to do with the overall story.  It dragged because they spend whole minutes of the show on dead time. Such as, walking, opening a drawer, looking perplexed or sad...things that did not need to be filmed for that length of time. I have no idea how they managed to make solving murder mysteries draggy but they did.  I get it.  If they did not linger around a bit the entire show would end in 10 episodes. After all, the entire show focused on maybe 4 or 5 cases? It really tested my patience because I found myself screaming "walk faster," and " come on, get to the point..." a lot in this show.

Rather than spend whole minutes on dead time, this drama would have benefited from more light- hearted scenes. There was some humor but seriously NOT ENOUGH. There were a lot of dark themes in this drama.  They could afford more scenes of So Hyun being a girly girl in a squad full of dudes.  I rejoiced watching So Hyun's pink bowed, pretty girl past. I wanted to watch more.

Another con was the plot holes.  SPOILERS AHEAD. So I get that our heroic duo could alter the past but keep things consistent.  Park Haeyoung remembered things that has changed in history.  At first, So hyun doesn't remember anything, but as soon as she gets in on the walkie talkie business, all of sudden, she remembers what changed too?  The heck?!  Also is every cop in this fictional world this incompetent or this corrupt?  Aside from Jaehan, do these "cops" even do any active police work?  Everyone just sits around and  it seems like Jaehan is the only one running around looking for evidence.  Which is frustrating and convenient because half of the drama is simply due to incompetence and over fuckery from people not doing anything. Where the hell is his "team?"

Deaths also lose all relevance and actually become annoying when those same people come back the next episode. It's like, man, I actually felt bad the first time. Do it three times and it's annoying.

The last episode was great but the last 20 minutes made me want to punch a wall.  Don't give us an euphoric conclusion with well deserved closure and then take it all away with another random tangent plot point that NO ONE cared for... Can you sense the frustration from me? UGH. END SPOILER.

This was the longest review I've ever wrote but I guess I felt like I needed to justify why I gave a lowish review for such a highly rated show.