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Do Min-woo, a professor at Hankuk University who has just gone through a divorce, agrees to look after Hyung-jun's son, Yoon-ho, at Hyung-jun's request-a man for whom Min-woo once harbored special feelings. As Yoon-ho moves into Min-woo's home, his unrestrained behavior begins to unsettle Min-woo, and "Article 101," which prohibits romantic relationships between professors and students, stands as a barrier between them.

After sharing an unexpected kiss with Yoon-ho, Min-woo tries to maintain a distance, mindful of their professor-student relationship, but Yoon-ho, on the other hand, actively pursues her. Meanwhile, as efforts to uncover allegations of past inappropriate relationships on campus and the hidden truths behind them come to light, a suspicious atmosphere begins to linger over the once-peaceful school.

Upon entering Min-woo's lab, Yoon-ho runs into Park Seung-heon, who harbors special feelings for Min-woo. However, as Seung-heon suddenly disappears and a series of alarming signs follow, it becomes clear that Yoon-ho has another motive for staying by Min-woo's side: to uncover the truth behind his brother's death.

Seung-heon confides in Yoon-ho about his past relationship with Min-woo and the fact that Chi-won disappeared shortly afterward. Yoon-ho, who had suspected his brother was missing, confronts Min-woo about the truth. Upon hearing that Chi-won had committed suicide, he decides to believe him, but when it is revealed that Chi-won is actually alive, it foreshadows yet another secret.