The plot is quite innovative in a way, we don't have many series that tell the story of a couple, one of whom has a dark and bloody past, and the other is a detective who investigates a story that seems be related to her husband and his past.
So, yes, the plot is really well thought out.

And I'm not going to lie, the acting of the characters is really good, especially the heartbreaking scenes of FL which pleasantly surprised me by being determined, and at times manipulative to find out the truth.
I've seen others ML acting better than in this series, I think his emotionless face was realistic but it was more when he was emotional that I struggled.

In reality, the fault was the script, and the tone of the series, which for me was too light. The ML's actions were sometimes slow, hesitant, and for a person not supposed to feel emotions, he displayed way too many. He wasn't cold enough, not convincing enough, and that's not because of the acting of the ML overall, but because of the way he was written.

And although the producers surely wanted to make the drama as realistic as possible in the sense of a psychological detective series, the colors and tones were not scary enough, the impression of the series was not punchy and dark as in "Mouse".
The intensity quickly left as well as my attention, I finished the series just to finish it but well, as soon as the revelations of the past of ML were made - which were disappointing, in my humble opinion - there was no more addictivity to this drama.

So honestly, with this series they could have made a mind-blowing psychological thriller, with the intensity rising, the couple suspecting each other, the deaths continuing and increasing the pressure for the audience.
But on the contrary, the FL discovered her husband's identity very early - too - soon, then the romance came together to make the ML soft, and the intensity disappeared.
Frankly, I think the potential of what this series could have been left me with a bitter taste, the problem was the producers who desperately wanted characters that were bad but not too bad, to tell a narrative so that we gives sympathy for all the characters.
In short, disappointed with the direction that the plot has taken.

Special mention nevertheless to Kim Ji Hoon, who will soon play in "Money Heist : Korea", he was the only character really disturbing by his intensity, his darkness, frankly the acting of this character was perfect for the vibes of a series of psychological thriller, pity that the script and the tones of the series does not match. He would have been perfect in "Mouse" or "Beyond Evil".

I'm going to end it here, so I think it's a pretty enjoyable drama, the love story is believable but takes it a bit too much of the overall vibes, and makes the characters too soft, so the script is good, no inconsistencies, just not mature and dark enough for my taste.
I recommend it for those who like enjoyable thrillers with a strong romance, however those who really want something dark, very intense with suspense, this may not be for you.