Well, I'm pretty mixed on this series and I might be one of the few people who think this way.

This is the story of Oh Hyun Jae who used to be a top criminal profiler, but one day an explosion, caused by a serial killer, ended the life of his fiancé. After the horrific incident, Oh Hyun Jae vanished and he now lives in seclusion. Meanwhile, Cha Soo Young is a police officer in the countryside. She has a special ability which allows her to remember exactly everything she sees. Due to her special ability, she is selected as a rookie detective who will partner with Oh Hyun Jae informally.

For starters, I think the show seems a bit all over the place with the story overall, like the script, the plots and sub-plots, too much estranged. Also, the killers have little light and they're pretty much cartoonish - the insane laughter, the grandiosity, the wish to be the best.
As for the "main" killer, I figured out who he was basically three or four episodes before he was revealed, honestly it wasn't hard and the twist wasn't there. But he was the most interesting character of the whole cast, because he was precise, calm and intelligent.
By the way, the killers aren't the only ones too theatrical, the character Oh Hyun Jae was too, too dramatic, emphasizing a lot on his "mystery", like over the top, really playing too much on the behavior, the intonations. Also his monologues which explained the whole situation were frankly cringe and moreover, it does not let the audience think.

Their superiors are self-centered jerks, really it's for the biggest surprise that the female lead, actress Cha Soo Young is the one who saves the drama, her character is relatable, she tries to do her best but doesn't get enough light, and I have found that her colleagues are also the nicest to watch and her relation to her father.

I really feel like overall this series was empty, very little realism, far too much fiction. I couldn't empathize with Oh Hyun Jae, even for his profiler character while I love profilers, also he made me feel like he was more a mentalist than a profiler. The story is based on this killer and the truth of the events that took place 5 years ago, but the show really lacks substance and flavor. As for the end, the epitome of cringiness.

A disappointment to be honest, because the actor who plays Oh Hyun Jae, Jang Hyuk was excellent in his role in Voice, so I expected a lot but he missed the boat. I really finished the series to finish it, but god it was slow, messy and boring, the bad acting of most of the actors really didn't help.

Afterwards, I'm sure that the series can appeal to others, it's quite pleasant to watch - overall - but personally, I haven't seen anything in particular that would make this series stand out.
So do I recommend ? I would say yes, some people will probably find this series fantastic to discover and I think it can fill the time if you are bored, but hey, it's not a masterpiece or a chef d'oeuvre, I have seen a lot better in terms of thrillers produced by South Korea.