What are some of the issues with the plot? These are plot holes that were just too convenient, inconsistent or flawed. Off the top of my mind, the issues I had included:

  • No one could find Do Hyun Su even though he was working at the restaurant with his actual name with his normal looks?
  • Baek Hee Song just happened to hit Do Hyun Su - some many years after the incidents?
  • Baek Hee Song never committed any further crime between when Do Hyun Su disappearing and hitting him with the car?
  • Baek Hee Song was on the way to meet Jung Mi Sook - some three years after?
  • It was took convenient that the person who hit Hyun Su was the accompliance.
  • The rationale that the Baek parents would allow Hyun Su to adopt their sons name doesn't stand up to scrutiny.
  • Baek Hee Song was a maths genius, but he didn't use any of those skills in the drama? Hyun Su didn't study maths but continued his metalworking craft? The Baek parents weren't concerned that their son all of a sudden changed his talent?
  • Similarly their coldness towards Cha Ji Won doesn't stand up to scrutiny either - its been 15 years since Hyun Su has stood in for being their son, there's no need to be so on the edge anymore.
  • Where did Jung Mi Sook go after that day when Baek Hee Song hit Do Min Seok?
  • The voicemail call simply didn't make sense as it was left some months after the kidnapping so whilst the ice chipping sound could possibly be in the recording (as that's something that could have also occurred at the bar months later), the question asked by the bar tender could not have.
  • How did Park Kyung Chun know about the brand of the dog leash? His wife's body was never found and the brand was not publicly disclosed in relation to the other deaths?
  • What happened to Park Kyung Chun? Hyun Su egged him on to find the real accomplice but then he just disappears from the face of the earth?
  • The relationship between Hyun Su and Moo Jin is odd. Moo Jin dated his sister but also bullied him and was essentially his only friend?
  • How did Hyun Su get the money that Nam Sun Gil fought him over? What was that about? This was before he had met the Baek parents.

These are just some of the issues I recall off the top of my head but I'm sure there are more. 

I am of course willing to overlook all of these due to the amazing acting and suspense that the drama brought out. 

Plot holes were visible, but some of the ones you listed I don't think are plot holes.

They didn't even like Hyun Su, they wouldn't have liked any SO he would've picked, maybe being a cop made them hate her a little bit more. 

iirc Park saw the on the news about the method that Do Min Seok used to kill the victims, but they didn't put the real method publicly, so they knew he was a fake. 

He had money because he worked? In the last episode, it was told that he also had an inheritance from his dad. 

Moo Jin bullied him after it came to light that his dad was a serial killer, they were friends before that.

Park is in jail. Would've been nice to see him reconcile with Mi Sook. 

Jung Mi Sook escaped and was off wandering, we saw her in a tunnel. 

 largegiraffe:

Plot holes were visible, but some of the ones you listed I don't think are plot holes.

They didn't even like Hyun Su, they wouldn't have liked any SO he would've picked, maybe being a cop made them hate her a little bit more. 

iirc Park saw the on the news about the method that Do Min Seok used to kill the victims, but they didn't put the real method publicly, so they knew he was a fake. 

He had money because he worked? In the last episode, it was told that he also had an inheritance from his dad. 

Moo Jin bullied him after it came to light that his dad was a serial killer, they were friends before that.

Park is in jail. Would've been nice to see him reconcile with Mi Sook. 

Jung Mi Sook escaped and was off wandering, we saw her in a tunnel. 

Thanks for clarifying some of these issues. Just with Jung Mi Sook, it seems weird then that she never made contact with her husband following her escape, was never caught/identified by the police, even at the mental institute but was somehow recaught by Yum Sang Chul? 

Jung Mi Sook lost her memory after escaping (it's not explained how), we can only assume it was due to how traumatic the incident was. You can label this as another plot hole, maybe they just live in another universe where these things are possible, or imagine that she gets hit in the head. Another example of this was when Yeom Sang Chul died from sniffing rat poison in what I would assume is under 30 minutes. You kind of have to imagine that they live in a different world or it was a different kind of poison for it to even be remotely possible. 

I try to answer. :)

No one could find Do Hyun Su even though he was working at the restaurant with his actual name with his normal looks?
He burned their house and there was no actual photo records of him left. Police do not have his photo (couldn't flash it in TV news etc), remember how they rushed to the old woman when she said she had that one photo. He also was not a 'legal' worker at the restaurant, he has no ID and the owner pays him in cash. He (owner) was okay with it as he pays him cheap. The name I assume is also common.

    Baek Hee Song just happened to hit Do Hyun Su - some many years after the incidents?

THIS ONE I AGREE. It bugged me for sometime too. But I'm just gonna attribute it to the supernatural or that minuscule chances of things happening.

    Baek Hee Song never committed any further crime between when Do Hyun Su disappearing and hitting him with the car?
All the crimes he did prior to that were done with DoMinseok, he was like a trainee with his master. He probably didn't feel confident enough to carry out another murder on his own yet. And in a twisted way Dominseok's death affected him too, remember how emotional he was at the funeral.

    Baek Hee Song was on the way to meet Jung Mi Sook - some three years after?
Yeah, this one is also questionable. Either she went crazy wandering around for 3 yrs before they found her again. Or when DoMinseok died Heesung sent her to the mental hospital (maybe DoMinseok's death bothered his killing streak), then she escaped the institution and got caught again on that tunnel and Heesung finally making up his mind on what to do with her (that night he hit Hyunsu).

    It was took convenient that the person who hit Hyun Su was the accompliance.
YEAH. I initially preferred if the Baeks had nothing to do with the serial killing. But anyway, the writers managed.


    The rationale that the Baek parents would allow Hyun Su to adopt their sons name doesn't stand up to scrutiny.
I also thought about this a lot. It seems there is the issue of the military service or being an army reserve. They couldn't say Heesung went abroad as that will be desertion. If they say he got in a coma because of an accident, it would also look bad on the Father as a doctor who couldn't cure his son (yes, korean society and their obsession for reputation). He has high position in the hospital. What bothers me is that what do they even plan to do once Heesung wakes up, he could have woken up at any point or so much earlier.


    B
aek Hee Song was a maths genius, but he didn't use any of those skills in the drama? Hyun Su didn't study maths but continued his metalworking craft? The Baek parents weren't concerned that their son all of a sudden changed his talent?
We never really knew other details on Heesung when he is not in his Psycho mode. But he seems pretty normal on the outside, even does volunteering, sweet with his Mother and such. I assumed he had some sort of profession going on too. BaekManwo asked Hyunsu to slowly cut ties (via email I assumed) with the people Heesung knows.
It would be more problematic if Hyunsu took up the same profession as Heesung's, it doesn't serve any purpose. First, obviously Hyunsu doesn't have the brains or the skill for it lol and second it would be problematic to do something in the field where people know the real Heesung.
And as for being a metalcraftsman, BaekManwo commented that Hyunsu doesn't like showing himself too much to people. It's a perfect for him also in the sense that he doesn't have to work with others.


    Similarly their coldness towards Cha Ji Won doesn't stand up to scrutiny either - its been 15 years since Hyun Su has stood in for being their son, there's no need to be so on the edge anymore.
It wasn't that they were extremely cold to Jiwon, it was Hyunsu who actually didn't want Jiwon to get close to his fake parents. In the scene where they attended his birthday, he called his fake Mother afterwards and told her to call Jiwon and be harsh to her so that Jiwon wouldn't expect a relationship with them (fake parents). Works for all of them so they keep the secret intact too.
But also as confirmed in the last episode it is also because Hyunsu cares for Jiwon even then and somehow doesn't want to immerse Jiwon so much into the charade.

    Where did Jung Mi Sook go after that day when Baek Hee Song hit Do Min Seok?
Again we don't know if she goes insane wandering around or Heesung temporarily put her in the mental hospital before figuring out what to do with her.

    The voicemail call simply didn't make sense as it was left some months after the kidnapping so whilst the ice chipping sound could possibly be in the recording (as that's something that could have also occurred at the bar months later), the question asked by the bar tender could not have.
I forgot the question, was it the 'do you want an ashtray?'. yes, this was way after Do Minseok's death and the Village Foreman's death. I think it is not a question to Dominseok (he was ofcourse dead then). Maybe directed to Heesung if he smokes or some other customer?


    How did Park Kyung Chun know about the brand of the dog leash? His wife's body was never found and the brand was not publicly disclosed in relation to the other deaths?
Wasn't it published in Moojin's article? He got the other undisclosed details wrong though.

    What happened to Park Kyung Chun? Hyun Su egged him on to find the real accomplice but then he just disappears from the face of the earth?
Yes. Too bad they didn't put a reunion scene with JungMisook. Or probably he already died. He has terminal illness and had a few months to live from that time he kidnapped Hyunsu. There's not much he can do anyway with his situation. He just had to keep Hyunsu's secret so he can find the truth for him.

    The relationship between Hyun Su and Moo Jin is odd. Moo Jin dated his sister but also bullied him and was essentially his only friend?
Yes. They were friends before that, but Moojin turned his back on the siblings once the news was out that their father was a serial killer.
Also doesn't help that the villageforeman nephew is spreading that false rumor of Hyunsu killing the chickens and being possessed. Moojin probably got pressured or influenced by the other kids to bully Hyunsu.  Teenagers. LOL
But atleast Hyunsu kicked his ass in that basement.haha

    How did Hyun Su get the money that Nam Sun Gil fought him over? What was that about? This was before he had met the Baek parents.
He was saving money from his earning to open a metalcraft shop. And I assumed he grabbed some money from their house before burning it, turned out Dominseok was also rich. Haesu payed all the victims' family but somehow she and her brother still had inheritance left.

I have to add another unexplained thing too. The way the police ruled out that Dominseok committed suicide when his body was obviously smashed by Heesung. I wonder how Heesung managed to make it look like suicide.

I'ma label number the points you made, but I've noticed I disagree with your stance on a lot.

1. 

 PapaET:
No one could find Do Hyun Su even though he was working at the restaurant with his actual name with his normal looks?

No one was looking. The investigation was incredibly mishandled and although he was put on the wanted list, nothing was really done about it. Add the fact that not many people outside the town had heard about the serial killer I don't they properly advertised that a boy wanted for murder was out there. Also the restaurant was in seoul, or at least in gyeonggi, which is apparently hours away from his hometown by car, not likely the news traveled that far.

2. 

 PapaET:
Baek Hee Song just happened to hit Do Hyun Su - some many years after the incidents?

Not a plot hole, a plot contrivance. Although it's incredibly unlikely to happen, it doesn't create any narrative issues, only makes it feel fake/forced.

3. 

 PapaET:
Baek Hee Song never committed any further crime between when Do Hyun Su disappearing and hitting him with the car?

It's not that he necessarily never committed a crime again, it's that if he did he wasn't caught. also his last acts before laying low were pinning everything on the Do family, he was trying to shed the uneasiness from the possibility of being caught. The reason he didn't personally want to kill mi sook at first was because that lays more consequences on him should the duo be caught. Also, he was probably doing his military service for 2 years of that.

4. 

 PapaET:
It was took convenient that the person who hit Hyun Su was the accompliance.

Same as 2, not a plot hole.

5. 

 PapaET:
The rationale that the Baek parents would allow Hyun Su to adopt their sons name doesn't stand up to scrutiny.

Which part? They were trying to cover their sons tracks and prep to pin everything on hyun su should hee sung wake up... which is exactly what they did...

6.

 PapaET:
Baek Hee Song was a maths genius, but he didn't use any of those skills in the drama? Hyun Su didn't study maths but continued his metalworking craft? The Baek parents weren't concerned that their son all of a sudden changed his talent?

He earned awards as a kid, but a lot of people who are famous for something as a kid don't actually stick with it, for example Mara Wilson and Zach Tyler Eisen, both incredibly famous for their acting work as a kid but practically dropped off the map when they got older. If I remember correctly the former became a writer rather than being on screen. Another very common thing is people finding an art form they like and doing it later in life. My glass blowing teacher didn't start glass until she was 35, my pottery teacher when he was 41. One of my friends started pottery in her thirties and began selling her stuff while in school, she makes a pretty good living off of it too.

7. 

 PapaET:
Similarly their coldness towards Cha Ji Won doesn't stand up to scrutiny either - its been 15 years since Hyun Su has stood in for being their son, there's no need to be so on the edge anymore.

It's not about him, it's about their son and the fact that Jiwon is PUSHY. Most people don't have relations with in laws if they're that rude, but Jiwon went out of her way to have get togethers. The year previous to the drama starting they showed up at the baek house unannounced.

8. 

 PapaET:
Where did Jung Mi Sook go after that day when Baek Hee Song hit Do Min Seok?

Stress and trauma induced memory loss, she ended up in a psych ward judging the ep where she is reintroduced.

9.

 PapaET:
The voicemail call simply didn't make sense as it was left some months after the kidnapping so whilst the ice chipping sound could possibly be in the recording (as that's something that could have also occurred at the bar months later), the question asked by the bar tender could not have.

Oh, I actually don't quite remember this so I'd have to rewatch to form an opinion. any idea which ep this comes up in?

10.

 PapaET:
How did Park Kyung Chun know about the brand of the dog leash? His wife's body was never found and the brand was not publicly disclosed in relation to the other deaths?

He used to go digging in that town everyday. From episode 5 it's clear that the townsfolk know a lot about the case, with the doctor even knowing they suspect DHS could be an accomplice cause he was his father's alibi. It's not even a contrivance to say he probably heard the gossip.
 

12.

 PapaET:
The relationship between Hyun Su and Moo Jin is odd. Moo Jin dated his sister but also bullied him and was essentially his only friend?

Met in middle school, was his only friend, started dating his sister, turned on them after their father's death. Pretty straightforward.

13.

 PapaET:
How did Hyun Su get the money that Nam Sun Gil fought him over? What was that about? This was before he had met the Baek parents.

DHS was paid $300 per month for 3 years, all while being boarded at the chinese restaurant he worked at. he didn't have a life outside that job, didn't have to shoulder boarding costs, and probably didn't have to worry about food either since he also cooked there. if I remember correctly he also would sometimes work on pieces he could have sold some of those.

11. (missed this one)

 PapaET:
What happened to Park Kyung Chun? Hyun Su egged him on to find the real accomplice but then he just disappears from the face of the earth?

2 month to live diagnosis, and prison. The only time he is mentioned afterward is moo jin went to see him for info about his wife.


 usernamenotfoundXX:
I wonder how Heesung managed to make it look like suicide.

Me too, the conclusion I came to was that he threw the body off the cliff featured at the end of 15 that DMS mentioned. the cliff would also be a preferred suicide spot for people in the area, so knowing how cruddy that investigation unit was they probably went in with the assumption it was suicide.
I wish they would have explored that more because it was kinda a key point for the first 5 eps and having to speculate so much about it is ridiculous. The fact that his kids just accepted the suicide ruling too...

For the making it look like a suicide, I assume he pushed the body off of that very steep and rocky cliff. That would make sense because your head would be assumed to be all sorts of smashed in as your hit the rocks going down.