Episode 2 of "S Line" transports us to the hallowed halls of academia, where we are introduced to Kyu Jin, whose automobile seems to require a collective student effort to budge, or perhaps she's merely navigating the school grounds at a glacial pace. Detective Han's niece, a fellow pupil at this institution, finds herself pondering the statistically murky waters of teenage sexuality, believing the actual figures to be significantly underestimated; even she harbors her own unspoken boundaries.
Back at her abode, Hyun Heup discards the cardboard covering her windows and embarks on a tidying spree. However, the tranquility of school life is soon disrupted as Seon-A squares off against her bully, only to be separated by Kyu Jin at the eleventh hour.
Reprimanded for the altercation, Seon-A prepares to depart for the day. Before exiting, she discovers a pair of glasses in her locker, which could prove to be more than just an optical accessory.
Meanwhile, at the precinct, Detective Han Ji-Wook remains engrossed in the case, sensing an underlying discrepancy. Driven by this unease, he revisits the murderer's lair, where a daunting wall adorned with countless red lines greets him. Meanwhile, at home, Seon-A dons the glasses, revealing a line hovering above her own head, instilling her with a sense of foreboding. Upon Ji-Wook's return, her尖叫 echoes through the house at the sight of the multitude of lines. As she ventures out, nearly every passerby is marked with one.
The following day, Seon-A wears the glasses to school, keenly observing the interconnected web of lines. Her eyes catch a particular connection between a teacher and a student, sparking intrigue. Amused to find her homeroom teacher, Kyu Jin, devoid of any lines, she experiences a moment of bewilderment. The day takes an unexpected turn when Hyun-Heop enters their classroom as a new transfer student.
This episode weaves a tapestry of mystery and introspection, painting a vivid picture of a world where lines not only delineate but also reveal the hidden threads of human experience.
Still engrossed in the investigation, Ji-Wook notices the killer's distinctive glasses in one of the photographs but fails to locate them within the evidence box. Determined to uncover more, he makes his way to the school and catches up with Kyu Jin in the office, inquiring whether Jeong Min had a habit of wearing glasses.
Meanwhile, atop the school roof, Seon-A confronts her bully with newfound courage, only to be kicked again with harsh force. Hyun Heup intervenes swiftly, employing decisive moves to halt the assault. After a brief and flirtatious exchange, Ji-Wook continues with his next clue, a bar frequented by Jeong Min. According to the bartender, Jeong Min was known to indulge in drinks while counting under his breath, once even engaging in a heated altercation with another patron. Intriguingly, the bartender reveals that contrary to the crime scene headshot, Jeong Min used to wear glasses.
On her way home, Seon-A catches sight of her bully entering a hotel with a teacher and promptly sends her a snapshot. Bully Hye-young responds flippantly, insisting that any repercussions would fall solely on the teacher. The next day, the teacher receives the same image accompanied by a blackmail note demanding 20 million Won in cash.
As Seon-A walks home, she ponders whether 20 million Won would suffice for an apartment deposit – her desire to move out burning with anticipation. The conversation veers towards their shared lack of parents, prompting Hyun Heup to wonder if her mother is still alive.
A fresh day dawns at school with a photo making the rounds and a new bully making his presence felt. Hyun Heup confronts Seon-A about seeing ominous red lines through the glasses and admits she too can see them. However, Seon-A refuses to heed the warning about the previous owner's fate.
Now that the teacher has discovered it is Seon-A who is threatening him, he discreetly conveys through a message that the money is ready. Meanwhile, Seon-A has an appointment with the boyfriend of her bully. Unbeknownst to them, Hye-yeong eavesdrops on the conversation between Seon-A and Hyun Heup, learning about the glasses. Seizing an opportunity, Hye-yeong trails after her and slyly snatches the glasses, leaving Seon-A to mistakenly believe that Hyun Heup is the culprit who took them.
After falsely accusing Hyun Heup, Seon-A hurries to the school to confront the teacher and collect her demand. The teacher, attempting to wiggle out of the棘手situation, receives a text from his wife containing a photo she had recently obtained—exposing his deception. Just as Hyun Heup arrives on the scene, a harrowing sight unfolds: Seon-A plunges from the rooftop, fatefully meeting her end before his startled eyes.
