Initial review: Great start, it's been a while where I've been this hooked to the storyline and presentation in a show. The way the story progressed through this 2 episodes, is simply, marvelous. The suspense buildup, has been, just WOW, that's how you build suspense in a show.
Also, and this is important, I would really like to thank the cinematographer for his/her masterful work. The cinematography goes so, so Crazy Freaking Well with the storyline, the characters portrayed and also the overall vibe of the show. The music is just perfect for a story like this, very well done. This is the direction which Kdrama should move towards for suspense stories, as far as I'm concerned. So far so good, let's see where it goes. After I've finished the series completely, hopefully will be able to write a detailed review.
When it comes to the cast, with Lee Sun Kyun (My Mister, Parasite), Park Hee Sun (My Name, Missing Noir), Seo Ji Hye (Dinner Mate, Crash Landing on You) and Lee Yoo Young (Tunnel, Your Honor), the cast couldn't have been any better. For now, I would suggest everyone to give this one a go. So far, it's really really good.

Final Review: It's been a very good experience for me watching this drama. Although there were moments where scientifically a few things didn't make sense. But since it's not as much of a science-fiction but rather more of a fiction, I will give those a pass. Overall, good show, and quite a unique one.
Now, I can say confidently that it's not a drama for everyone. Many elements that makes a K-drama popular in the generic K-drama fanbase were absent in it. So, if you don't feel like it's your cup of tea, then maybe this genre simply isn't for you.