Hye Seong's Father's Notebook (S01E06)

After the school's security guard passes away, Hae Seong discovers a notebook that belonged to his long-missing father. The unexpected find sends him into a fury, leading him to confront the NIS for clarity. The information he receives, however, causes him to withdraw from the mission and take a break. Despite this, he decides to press on, only to find out that Su A has seen everything unfold.