After watching this spin-off, I suggest that you should watch the original, The Veil, as one of the huge pros about this particular series is that you kept in the dark as Han Ji Hyuk too finds out what happened within the year that he lost his memory.
I feel like this could have been longer to explain PHS’s character’s hostility towards NGM’s character with situational evidence and that this could have been the finishing piece to complete the full storyline as there are still a lot of questions within me that I’m unable to answer. However, despite this, I still enjoy the spin-off even though the action scenes are less intense than the original due to the lack of time to build up the suspense. The chemistry between the main characters is so strong despite it not being romantic is refreshing. Watching this finally allowed me to connect some of the loose strings together but I wished it was longer. But I do think that this might be one of the best ways to end a drama: keep them wanting for more.