I'm gonna be objective with this review STORY: The story is about 2 different cops from different timeline, connected by a single walkie-talkie. Once they finally realized this is happening for real, they decided to stop multiple unresolved serial murder with present evidence/locations of the murder to stop it from ever happening. in all honesty, this is actually a great concept of a story that you rarely sees in asian drama..The story progress was smooth and develops very well towards the end. They used a lot of flashbacks from what they've already shown you in previous episodes but these flashbacks actually hide tiny details that you tend not to catch on your first watch...So if you're worried that it might bore you with all the flashbacks, don't...it's rather good execution.. ACTING - Superb acting by the man who plays Lee Jae Han, you actually buy it that how anyone of his character would react...Also the female lead officer did superbly as well... there's multiple times that I actually felt it was real thing that is happening, not an act--like the person is actually experiencing the traumatic events/heartbreaking events for real... OST - I have them in my music collection and I play them regularly even after almost 2 years...that should tell you how it was REWATCH VALUE - You don't get bored with the story, the acting, music, the humanity that this drama brought... even if you're not interested in crime drama, you should give this a try I've watched this drama from start to end 3 times so far, first time alone, second time with my dad which we watched until 4am and third time, with my mum whom always gets emotional in some of the scenes.. The only issue I had was with the ending...I thought it could've done better way..For me, it had barely any closure on the characters...some might argue it warrant a sequel, which I doubt it happening