I don't know where to start T_T The plot is really good. It is impossible to find any plot holes in this drama. The parallel theme is utilized in this drama really well. I have watched other dramas with parallel/time travel theme like King Eternal Monarch, 365: repeat the year, Alice, and Kairos this year. Personally, I think this drama is the best in terms of plot and acting. There are a few good plot twists at the end. It reminds me of the dramas Signal and Tunnel although they used time traveling theme instead of parallel. The acting of the male lead is extraordinary. Other actors are also good, but I am extremely impressed with Yoon Shi Yoon's acting in this drama. Playing double seems to be his thing. Do Won A and Do Won B 's acting gave me goosebumps. The OST is perfect for the drama. With you (Dowon and Sekyung's theme) is my fav song. You should definitely give it a shot. Definitely drama of the year!!!