It is definitely one of the best among mystery/sci-fi genre in K-drama world. With the short length of the drama, the story was very tight and well-paced throughout the drama. Only the first episode was a bit confusing because it introduced the characters and set up the plot. The fact that it could manage to show and link the two parallel timelines in each episode, without exposing any major plot for the future, is very impressive, salute to the writer. Story wise, it is definitely my cup of tea - bromance and sci-fi and thriller - kind of present and future linked together in a way. The bromance is real and epic and you can feel both brothers' emotions when they are fighting hard for the other. The twists and turns are quite unpredictable that makes you keep guessing. Romance-wise, I quite liked Woo-jin and Jung-yeon's chemistry. Although they had little chance to develop it further, their romance is quite fresh and innocent, amidst the mystery and dark happenings around them. Acting-wise, everyone played their characters very well. Although I have never had much interest in Yeo Jin-goo in his earlier dramas, this character Woo-jin, kind of made it for me. He portrayed Woo-jin as a 20-something boy with a burden to carry but also with a boyish charm. And for Gong Seung yeon, it is my first drama watching her and I think she portrayed the three characters very well and I am looking forward to see her in next dramas. And lastly, Kim Kang-woo, as a more experienced actor than the other main leads, nailed his character perfectly. Although he seemed more mature and calm and calculative, compared to his younger counterpart which makes sense after all since so many years have passed, sometimes he was as wild and crazy as his younger self. As for the villains, I have nothing much to say. They served their roles throughout the story. As it is a mystery genre that usually becomes a bit boring for a second watch, I don't think I will watch it anytime soon.