In Episode 6 of *S Line*, we find ourselves in a café where a young woman receives an intimate video that was never meant for her eyes. When she arrives at her university class, it's painfully obvious that she has become the day's hottest topic of gossip. Later, at home, she hides under her duvet, her sobs muffled by the covers as she continues to receive a barrage of threatening text messages. That evening, despair drives her to the brink, and she attempts to step into oncoming traffic—only to be stopped by Kyu Jin, who appears out of nowhere. Before departing, Kyu Jin, with an air of innocent intent, hands her a pair of glasses.
The scene then shifts to Hyun Heup, who waits patiently at the bus stop for Jun Seon. She approaches him with a request that unexpectedly lands them in a hotel room. After their time together, she notices the mysterious S Line connecting them on what seems to be an invisible plane. As they walk toward the bus stop, Jun Seon suddenly dashes off to fetch them some snacks. While Hyun Heup waits alone, she is startled to see her newfound S Line connection vanish before her eyes. She turns her attention to a commotion erupting nearby.
There, she finds Jun Seon, who appears to have metal rods protruding from his back in a grotesque display. Overwhelmed by a sense of foreboding and believing herself to be cursed, Hyun Heup witnesses another person close to her meet a tragic end. Next, she checks a video message on her phone, only to find her school friend tied up and a "Happy 17th" message flashing on the screen, along with an ominous invitation to the 'room of boundaries.' Desperate, she calls Detective Han for help.
Seated together in the car, Han's mind is still preoccupied with the horrifying implications of Seon-a’s S Line. Hyun Heup interrupts his reverie, explaining that someone has been counting down the days to her 17th birthday and that there are demands being made of her. She shows him the threatening texts as proof.
As Han listens, he begins to piece together the puzzle—how she was lured to the school, how he came to possess the glasses, and whether these events could be the work of the same individual. A chilling realization dawns on him as he notes that her teacher, Kyu Jin, doesn't seem to exist in any official records.
Together, they sprint into the school building, their hearts pounding as they race toward the brightly lit classroom. Hyun Heup spots her friend, who explains she'd come to meet her. Without hesitation, the girls dash back out, determined to find Han. But every time they take the stairs, a bizarre twist of fate sends them hurtling back to the same floor, as if trapped in a never-ending loop.
Meanwhile, Han ascends to the rooftop. Hearing the creak of a door opening, the girls follow suit. Yet, when they emerge, they find themselves in separate realms. Han approaches a cluster of people, one of whom remarks on how fortunate she was to have crossed paths with her. Memories flood back of a conversation where Kyu Jin had posed a chilling question: Did she still yearn for death, or would she prefer to be the one to deal it? The woman testifies that, with the aid of the glasses, she discovered a newfound purpose, erasing a line with each life she took. Revealing a man bound and helpless, she raises her knife, poised to sever her final sin and embrace rebirth.
On Han's rooftop, he spots Kyu Jin advancing, millions of crimson threads emanating from her head like malevolent veins. When her gaze falls upon him, she unveils Han's father, tied up and drenched in blood. Offering Han a blade, she taunts, wondering if he wishes to save Seon-a. In a desperate move, Han turns the knife on her, but she effortlessly wrests it away, plunging it into Han's father instead. As he fires at her, the bullets ricochet harmlessly off her as she continues her relentless march toward him, seizing him by the neck.
Back on their own rooftop, Hyun Heup struggles to make sense of the chaos unfolding before her. Then, she notices Gyeong Jin sporting the glasses. In a flash, Gyeong Jin's hands close around Hyun Heup's throat, but a fierce struggle ensues. In the heat of the moment, Hyun Heup smashes the glasses, and Gyeong Jin snaps back to reality. As she does, she can't help but notice the transformation in Hyun Heup's eyes—a subtle yet striking change in hue.
Droplets of blood jolted Hyun Heup's senses, scattering her focus as she found herself ensnared within a throng of people encircling Kyu Jin. He declared to her that her innate ability to perceive S Lines was nothing short of a divine gift. Suddenly, the crowd collectively removed their glasses, closing in around Hyun Heup like a suffocating embrace. Kyu Jin raised a lengthy sword, poised to strike. But in that critical moment, Hyun Heup's mother interposed herself, shielding her daughter and taking the fatal blow instead. Gasping, her mother confessed that she too had once worn the glasses, absolving Hyun Heup of any blame.
Regaining her footing beside her fallen mother, Hyun Heup charged at Kyu Jin, meeting the sword's edge head-on. Yet, her blood seemed to defy gravity, drifting away in delicate droplets. In a surreal twist, Hyun Heup rose again, her body emitting radiant lines that tugged her towards a crimson orb. After a brief, ethereal pause, she was gently deposited back onto the ground as the orb advanced upon them. As Kyu Jin watched in astonishment, Hyun Heup plunged her weapon into him. But she declared, with a mix of resolve and sorrow, that it was too late – their mission had already been accomplished.
Panic erupted among the crowd as their glasses vanished, including Han's. In an instant, Han, Hyun Heup, and Gyeong Jin found themselves atop a rooftop, Han's hand outstretched for his glasses only to watch them slip away. Below, on the streets, people stood transfixed, gazing up at a sky ablaze with red. And for the first time, everyone could discern the red lines connecting them all.
As dawn broke on a new day, the subway was filled with individuals donning glasses once more. Hyun Heup, with a hint of wistfulness, remarked on finally feeling "normal." Seon-a had awakened from her coma, reborn without an S Line to define her. And as for the rest, Hyun Heup found herself solitary once again. She visited Jun Seon's grave, a place of quiet reflection, when she heard her name called out – it was Kyu Jin, somehow still present in her life.
