Hyun-heup, a girl who was able to see the S-line that indicates sexual relations since birth. Because of this ability, Hyun-heup's family faces tragedy, and Hyun-heup becomes a reclusive loner for a long time.
Hyun-heup goes to Bangju High School after receiving a mysterious text message. There, he reunites with Seon-ah, a middle school classmate and Ji-wook's niece.
Seon-ah falls into a coma after a rooftop incident. Ji-wook, filled with guilt, works with Hyun-heup to find who's responsible. Jung-woo, a teacher, ends his engagement after seeing his fiancée cheat. Later, he finds glasses on his desk.
Ji-wook investigates Seon-ah's case and doubts Hyun-heup's S-line theory. Goshiwon resident Bang Ju-goo finds liberating S-line glasses at Mi-seong's, then feels jealous seeing a woman linked to his crush.
Ji-wook finds glasses in a bloody bathtub, confirming Hyun-suk's claims. Using the glasses, he investigates runway sexual assault cases and suspects someone linked to Sun-ah's S-line is behind her case.
