this review is not exhaustive by any means but i hope that it helps.

Let's start with the good parts about this drama:

- The twists: While it's true that I foresaw who the real culprit was a few episodes in, still, I had to admit that the twists were somewhat reasonable and believable. It's not that it will take you long to find out who the "culprit" was, it's just amazing in a sense that it's not who you would usually picture the culprit to be.

- Melodrama: Though it was kind of draggy at times, I really enjoyed the melodrama and romantic aspect of the drama, in no small part due to Yoon Shi Yoon's acting (he just gets the right expressions during these times, it's a treat watching him on the screen). I think he and Kyung Soo Jin (who, for those who don't know, played Song Si-Ho on "Weightlifting Fairy Kim Bok Joo") have real chemistry. It was fun watching their flashbacks on-screen.

- Cinematography: I can tell apart the two worlds and its characters within. The styling for the characters' costumes was spot on as well.

Now, having said that, we move on to the parts where the drama could have improved:

- /some/ badly written characters: Though the acting was good, I really thought that emotionally invested detectives/prosecutors are a bit of a caricature these days. For those of us who have been watching kdramas for some time now, we can all agree that we've seen that emotionally-driven detective or prosecutor at least once in kdramas. They're smart to an extent, but driven by their emotions to a fault (like when Do Won pushed Seok Min Jun against the wall, or when Seo Kyung pushed Do Won against the wall).

I hated how Do Won was written like a Mary Sue, who was upright, handsome, and all things nice. Oh and he's also short-tempered, but that flaw is just really annoying more than humanizing. I would have loved to see him become a little bit more relatable, because his character's become a Mary Sue caricature basically. Always doing what's /good/ without having real flaws.

Plus, what was with Han Seo Kyung's character anyway? Originally a no-nonsense, here-to-get-the-job-done persona-- and does not get repelled by Do Won A's advances (hugging her out of nowhere, touching her face with her gun to his chest, etc.)?! I smell inept male writers who can't write shit about women characters.

And what was that about Do Won's illness/ringing in his ears? I really don't see how that has any significance to the plot.

I think the drama could have benefited from stronger character writing (for example, the writers could test the characters' goals and compare it against their behavior: is it justified or is it far-fetched already? Personally, I just can't reconcile how being a psychiatrist justifies serial killings because of a personal trauma--It might have been the acting, but I'm really not buying how Min Jun's character was written, and so are Mi Sook's motivations for doing what she did. It felt so flat and one-sided because we couldn't even see her detective side being conflicted about her actions.)

The drama could have delved into the motivations of the characters as much as they did the backstories so we can really empathize with the villains and find them relatable. For example, for someone who pursued medicine as a psychiatrist, you would have thought that Seok Min Jun's character understood the importance of mental health and received professional help as well, so I don't really understand how he can become a serial killer as a psychiatrist. I don't mean to say it's impossible, just that I couldn't rationalize it given the way his character was written. You could literally make him into any other profession and it still would not change how grisly those murders can be. I think the writers just made him Seo Kyung's therapist so there'd be some surprise factor for the viewers. To realize that someone supposedly gentle was behind the murders all along. But I think even if you cast him as Seo Kyung's friendly neighbor--for example, that surprise factor would still be there). And for someone who mercilessly killed someone at 17, how do you even rationalize being a therapist?! How?! Exactly my point. The drama failed to show us that.

Lastly, yeah the last victim in the story was supposed to be Oh Mi Seok (the surprise factor is she is the serial killer's mom). So all the characters are frantic and in a hurry to rescue her before it's "too late", conveniently forgetting that Oh's a police officer herself and is adept at self-defense. But we're conveniently forgetting that part of her character because she's what? A woman? This is the same case for the way Do Won treats Seo Kyung (but in all fairness, Seo Kyung as a police officer still can't defend herself for shit -- like when she was attacked by her stepbrother in that abandoned public market scene)

- unnecessary characters: I would say the two assistant detectives in the story were pretty useless. If I had a dollar for the number of times they asked Do Won where he was going and Do Won consequently ignoring them and being cryptic, I could buy a house! Plus I don't remember anything significant being contributed by these two. I wish they were written to serve a bigger purpose other than to be fill-ins.

- underutilized character: Personally would have loved to see the two Do Wons cooperating because they had the same goals anyway, just that Do Won B was a hopeless junkie. But the writers pit the two of them against one another, which was annoying, to put it lightly.

- lack of attention to detail: wherein sometimes they don't use gloves whenever they're investigating, which might contaminate evidence (like when they used the card to swipe on Lee Sung Wook's belongings and Do Won storming into Oh Mi Sook's place to find /that/ photo).

Plus, the fashion styles (also personalities) of the two Do Wons being polar opposites (Do Won A: smart and casual; Do Won B: more upscale and pedigreed), you would have thought the characters from world B could have more easily picked up that there was something different about the two Do Wons. Alas, they didn't.

- some bad acting: I don't get why Kyung Soo Jin always had that worried, furrowed eyebrow look on her face for every scene. Jesus. It's so cringey watching her scene after scene you start pitying her. She was so one-dimensional I had to fast forward her sometimes. Shin So Yool was a better actress.

- unecessary monologues/lines: Lee Hang Na's Oh Mi Sook mostly (NV: "we have to find the body", "no matter what happens, I'll protect you"). There are probably still some more within the drama but I can't remember them all.

Overall, I think the show gets its message across: reason to never cross over into another world is because your actions can have consequences, even the smallest ones such as handing over an umbrella. Did I think it was good? Certainly a good 12 hours to spend. But do I wanna watch it again? I usually don't, so it's no surprise I think this only makes for a one-time watch. I think I can only really strongly recommend this to hardcore Yoon Shi Yoon fans because seeing him act is the only reason I started watching (and stayed watching) this drama.