Well, this is the story of a young man, Lee Hyun, a young profiler, who returns to Korea after years abroad - in the USA - and who is interested in an investigation and murders that are related to the death of his father and the disappearance of his brother by a psychopathic criminal, Lee Joon Young. Of course, he has little memory of that time and the fateful day, so he tries to recall his memory and solve the mysteries.
He meets an investigator, Cha Ji-an, with whom he will team up to find Lee Joon Young.

Well, the set-up is typical of lighthearted k-dramas, the atmosphere of the series is rather comedic and not very serious, the inspectors are all nice idiots who don't know how to do their job, the "bad guy" Lee Joon Young is smart enough and manipulates everyone as he wants. The FL is stubborn, clingy, acts like an annoying stalker but she gives off the same energy as many other FL, she does not stand out with her acting.
It's really a series in k-drama fashion : comedy, a bit silly, some emotional scenes, a clumsy and boring romance with a decent ML.
Honestly, it's more the plot that stands out here, not that it's extremely well executed but the starting idea is good, it's also the story of two brothers.
I would have liked to see what this series would have done with a more serious psychological thriller and crime k-drama production. It would have been a gem among the genre.

People have painted this k-drama as one of the best but the storyline is basic, without surprises, the course of the story has been gently followed, the characters are just there to complete and without particular acting.
Honestly this k-drama is just one of many, nothing special, that doesn't need re-watching, with empty characters and no complexities.

However, special mention to Park Bo Gum, who plays the role of Lee Min, the little brother who is a killer, his acting is the one that stands out the most and makes us think that he has a lot of potential.
I hope to see him again in a crime and thriller series with a better production and script to match his talent, and not a banal comedy k-drama made to not make the audience think too much.

The ending is just as banal and even bad, a killer is just released into the wild thanks to the kindness of some of the characters, and I wonder what kind of ending is that which is far from realistic. Killers may indeed never be caught but people never release a killer because "he promised" to change.
So they are "bad guys" but not too much because be careful, they have extenuating circumstances and sometimes they show human emotions. So bad but not too bad.
The killers here were very romanticized for the audience. In short, really basic k-drama.