Hyun-heup is born with the strange power to see special lines that reveal the intimate relationships between people. This supernatural gift brings misfortune to her family and condemns her to a life of isolation.

Hyun-heup goes to Bangju High School after receiving a mysterious text message. There, he reunites with Seon-ah, a 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.

Ji-wook discovers Sun-ah's S-line secret through glasses. Hyun-suk makes a major choice before turning 18 and encounters Jun-sun, causing confusion.