The premiere of "S Line" Episode 1 unveils a succinct exposition—Social, Sex, Secret. It condenses into the evocative term "Sex Line—S Line," symbolizing a crimson thread intertwining those who have shared intimate encounters. Hyun Heup pushes aside the cardboard shrouding her window, only to overhear a man in a vehicle, his head emanating a plethora of red lines, attempting to entice a schoolgirl into his car. Across the street, a woman indulging in vaping sports a few such lines emanating from her head as well.
Ensconced in darkness, Hyun Heup dons dark sunglasses, mourning the fact that some facets of life are best left unseen. She has borne witness to these since her birth. As she matured, the significance of these lines dawned upon her. Her childhood habit of sketching them drove her mother to the brink of madness, ultimately leading to her father's demise. She tried to shut out the world, keeping her eyes tightly shut and to herself.
Unable to end her own life, Hyun Heup now ponders the originator of these lines. She wonders who else possesses the ability to discern them.
Detective Han grumbles about the responsibility of caring for his niece, chuckling wistfully when a colleague teases him about what would become of her if he ever tied the knot. Unbeknownst to them, he is entangled in an illicit affair with another colleague on the brink of matrimony. Meanwhile, his niece, Seon-A, endures relentless bullying at school, the girls taunting her for presumably tattling on them to her uncle.
At a grim crime scene, Han meticulously probes a mysterious death, while Hyun Heup, in her own world, receives yet another basket of fruit from her enigmatic new neighbor. As she watches the evening news coverage of the local homicide, her eyes catch sight of an individual whose facial lines seem almost non-existent—a rare and striking feature. She spots him again across the street, initially mistaking him for the murderer due to his unusual appearance. However, she observes him being warmly greeted by her new neighbor, and a new line forms in her perception, connecting the two.
Elsewhere, Seon-A decides to skip school, later approaching her uncle for money to pay off her bullies. Just as Han hands over what little he has in his wallet, the bullies break into her locker. Noticing the bruises marring her face and arms, his concern deepens, but she remains tight-lipped. Smoking thoughtfully by the window, he spots Hyun Heup's apartment across the way, where she has partially removed some cardboard shielding her view. Coincidentally, a colleague calls to inform him of a police report filed in their neighborhood, prompting him to head over for an investigation.
Arriving at Hyun Heup's door, adorned with an intricate pattern of 'S' lines, he finds her hesitant to open it. A fellow neighbor remarks on her reclusiveness, hinting that she rarely interacts with anyone. However, she does eventually open the door, quickly explaining her mistake with an emergency call before abruptly shutting it again. Later, another deliveryman, bearing a single distinctive line on his face, arrives at the building, adding to the intrigue surrounding Hyun Heup's new neighborhood.
At the bustling station, Han and his colleague Dongzhi engage in a captivating conversation about the intricacies of coincidence, just as their curiosity prompts them to delve deeper, a distressing call interrupts Han's thoughts. His father is missing, a situation that leads Han to rush to the hospital, only to find him perilously perched on the rooftop. Meanwhile, Dongzhi, engrossed in uncovering clues, finds himself the target of a sudden and violent attack.
From the sanctuary of her window, Hyun Heup catches a glimpse of the mysterious figure with the distinctive one-line marking, his gaze directed towards her new neighbor's abode. Witnessing the attack unfolding, she hastily grabs a knife and her sunglasses, yet her fear immobilizes her, rendering her steps hesitant. Ultimately, overwhelmed by terror, she collapses unconscious on the pavement.
The neighbor, recognizing the assailant as Jeong Min, witnesses his attempt to choke Hyun Heup. As she manages to stumble into her apartment, clutching her knife, Jeong Min effortlessly disarms her and plunges the blade into her. Across the street, Han's sharp eyes detect Hyun Heup's apartment door ajar. Intrigued, he enters, noting her peculiar S Line board before his attention is drawn to the struggle occurring across the way. Without hesitation, he fires a shot, halting Jeong Min just before he could strike again. As Jeong Min slumps to the ground, his ramblings about vanishing lines echo eerily.
Hyun Heup, recovering, ponders whether Jeong Min's glasses bestowed upon him the unusual ability to perceive the S Lines. Her thoughts are further搅扰by a cryptic text message, its sender claiming to share her unique vision. Now, free from the confines of her home, the question lingers: Will Hyun Heup venture out to uncover the truth behind this intriguing message?
