Episode 3 of S Line transports us back to the harrowing scene where Hyun Heup witnesses Seon-A plummeting to the earth. They hastily convey her to the emergency room, amidst a flurry of urgency. Upon his arrival, Detective Han discovers Hyun Heup anxiously awaiting outside, though she remains in the dark about the circumstances, steadfast in her conviction that this was no suicide.
The following day, the tranquil ambiance of the school is shattered by the news of a missing teacher. Amidst staff conversations tinged with dread, Teacher Jung-woo, when prompted for a silver lining, reveals his decision to call off his wedding plans. Soon after, he arrives at his sister's abode, preparing to spend his final week there before relocating to Canada.
In the hospital ward, Seon-A lies lifelessly attached to a respirator, her uncle keeping a vigil by her side. Meanwhile, Hyun Heup discreetly scavenges for clues within the school's hallowed halls, while another pupil remarks on the absence of Hye-yeong, Seon-A's relentless bully. Hyun Heup's investigation leads her to individuals donning round gold-rimmed glasses.
Teacher Jung-woo stumbles upon a pair of such glasses at his desk and, upon donning them, is startled to observe crimson lines hovering above people's heads. Like everyone else, his heart races upon beholding the multitude of lines crowning Detective Han.
Han convenes with Teacher Kyu Jin to discuss his niece's plight and later meets with Ji-won, Seon-A's unhelpful and unconcerned date. Ji-won mentions that Seon-A dashed off upon realizing something was amiss. Han unearths CCTV footage depicting Hye-yeong, the bully, making off with Seon-A's bag.
Over dinner at his sister's home, Teacher Jung-woo unburdens himself about his break-up with his fiancée, who betrayed him with his friend. During a casual gathering with his brother-in-law, a chilling realization dawns upon him—his brother-in-law might be entangled in yet another affair within the very same building.
At the workplace, the police are eager to brush aside Seon-A's incident as a mere suicide attempt, catering to the school board's preferences, yet Han stands firm, locking horns with his superior. Inside the teachers' lounge, Jung-woo perceives underlying connections among his colleagues. He poses a hypothetical question to Kyu Jin: if she learned that someone dear to her was being deceived, how would she react? She cautions him to be absolutely sure before voicing any accusations, but admits she might also find amusement in observing the unfolding drama from afar.
When a package intended elsewhere lands at his sister's apartment, Jung-woo decides to investigate personally. Instead of sharing his findings, he and his sister delve into the bitter memories of their father's betrayal.
At school, a photograph featuring Hye-yeong and the absent teacher circulates among students and faculty. Han arrives at the scene of what appears to be 'another' suicide, where he reads a handwritten will. Based on the evidence before him, he concludes that the body was moved posthumously to the discovered location.
After classes conclude, Hyun Heup, unexpectedly mingling with her classmates, is approached by Han for a chat about his niece. She subtly mentions the glasses, urging him to find them, promising an explanation once he does.
Near the apartment complex, Jung-woo spots his supposedly travel-bound brother-in-law. Tailing him by taxi, Jung-woo discovers him dining with another woman, showering her with gifts. He phone calls his sister, feigning inability to reach her, and catches a glimpse of a hotel room key card lying on his brother-in-law's table, adding fuel to his suspicion.
At the hotel, Jung-woo confronts his brother-in-law, who is draped in a robe, vowing to sort everything out and reconcile with his wife. However, the next day, as Jung-woo packs his bags to depart, his brother-in-law has yet to make the promised call to his sister. Driven by curiosity and concern, Jung-woo cyber-stalks the other woman on Instagram and discovers to his astonishment that she is pregnant. Unable to reach his sister by phone, he rushes back to her apartment, only to witness her crashing into the other woman's car in a fit of distress. His brother-in-law arrives on the scene to comfort his companion, abandoning Jung-woo's sister to his care.
Enraged, Jung-woo snaps and punches his brother-in-law, only to be halted by his sister's cries for him to stop. To his utter disbelief, she reveals that she is, in fact, the other woman. And to add to the twist, this woman is already married to the man – blissfully unaware of her husband's secret double life.
Meanwhile, at the police station, Han is poring over the new victim's Will when he is struck by inspiration and races back home to inspect his niece's room. Finding her notebook, he ponders over the drawings of glasses and red lines, seeking clarification from Hyun Heup. She confirms his suspicions, explaining that with the glasses, one can see S Lines – a revelation that adds another layer of complexity to the unfolding mystery.
