The initial run felt seriously deficient across the board and essentially functioned as an opportunity for growth for those involved in creating the subsequent season. The way it depicted a cult seemed to lack any real, underlying purpose, instead seeming to borrow ideas from low-budget films that sensationalize cults—often pushing them into supernatural territory. These cults were exaggerated to such an extent that they appeared to have hypnotic abilities with just a few words, even affecting people who didn’t subscribe to spiritual or religious beliefs. Everything about the cult felt over-the-top, even considering it was supposed to be a dramatic portrayal. The storyline itself also had difficulty maintaining its own momentum, especially since it mainly centered on the viewpoints of those outside the group. It consistently gave the impression that something significant was being built toward, but that expectation wasn't fulfilled. Considering the length—sixteen episodes—it really needed more detailed world-building and a solid foundation for the cult itself. Giving the cult its own distinct personality would have helped establish a stronger sense of identity. Returning to the outsiders, having teenage characters as the main focus was another area where it fell short; they were uninteresting and underdeveloped. The mystery element, which should have been central to the drama, felt weak and didn't live up to what was promised. Honestly, the first season would probably have worked better if it had been a single movie instead of an ongoing series. I’m puzzled by its high rating on MDL, particularly because the second season successfully corrected many of the problems with the cult and provided a far more satisfying overall experience! The second season proved to be a much stronger offering than the first. Most importantly, it approached the cult theme in a sensible way. The characters felt more engaging and the casting was better than in the initial run; having older actors as leads was definitely an improvement. Uhm Tae Goo was a great choice for his role. The storyline progressed more smoothly and naturally compared to season one. However, things became rushed and less polished as it neared its conclusion. Speaking of the ending, I found it somewhat disappointing and not nearly as impressive as I'd envisioned. It really should have been presented as a completely separate drama from the first, even in terms of title. If you’re hesitant to commit to two seasons, you could just skip the first one entirely and watch the second on its own.

Source on TMDB