In Episode 5 of "S Line," Han finds himself lost in reminiscence, reflecting on a life where his ambitions were modest, his sole desire being freedom. Yet, for an extended period, he has shouldered the responsibility of being Seon-a's guardian. Now, at work with his glasses perched on his nose, he perceives the intricate web of lines and connections—or rather, the conspicuous absence of them.
Driven by a newfound determination to unravel the mysteries, Han persuades Ji Won to accompany him to the hospital under the guise of 'visiting' Seon-a. To their astonishment, they discover the absence of a red line connecting them, prompting Han to delve deeper into Seon-a's past romantic entanglements.
Meanwhile, at home, Hyun Heup engages in a lively text exchange with her friends, who have orchestrated a surprise birthday bash for her. The following day, her surprise turns to intrigue when she encounters a classmate adorned with four radiant red lines.
Han, engrossed in a report detailing a sexual assault case, hastens to the school to conduct his investigation. His keen eye notes the abundance of lines crisscrossing the student body, with the notable exception of Teacher Kyu Jin, who appears devoid of any such connections. Their interview with Han-yu yields little insight, leaving them at a dead end.
On her way to school, Hyun Heup catches sight of Han donning the mysterious glasses. He claims to be conducting a trial run to assess the glasses' capabilities, but Hyun Heup questions why such a task isn't entrusted to a forensic team. She chastises him for his casual handling of the glasses, now aware of their profound implications, especially in light of the recent crime linked to those very classes.
Kyu Jin intervenes upon hearing the raised voices, but Hyun Heup swiftly retreats. Before departing, Han, in a moment of awkwardness, extends an invitation to Kyu Jin for dinner, only to hastily retract it. As he walks away, it seems as though Kyu Jin catches a glimpse of his bouquet of S Lines, for she smiles warmly after him.
Continuing his relentless pursuit of the truth, Han pays a visit to Ji Na's entertainment company. However, he is met with claims that they severed ties with her due to her lackluster work ethic. They also casually mention her role as the instigator in encouraging other children to indulge in alcohol.
As they drove back, Dong Sik couldn't help but wish they possessed an infallible lie detector for this particular case, a thought that set Han's mind racing. Han then probed, asking if Dong Sik had detected any romantic interest in him toward that teacher, a claim Dong Sik swiftly and emphatically denied.
Turning their attention to the children, Dong Sik and Han employed a tag-team approach. Dong Sik questioned Han-yu intently while scrutinizing his phone, all the while Han engaged Ji Na and her mother in conversation. Han even conducted a small experiment, but to his disappointment, the metaphorical "lines" connecting them remained invisible.
In the neighborhood, Hyun Heup anxiously awaited Han's arrival, repeatedly checking her glasses as if they held the key to some mystery. When Han finally arrived, she confided in him that while their abilities might pose a danger, they also had the potential to bring aid to those in need. She mentioned how, just the previous week, Ji Na had displayed no S Lines at all. The detectives then hurriedly made their way to Han-yu's residence.
Meanwhile, Hyun Heup decided to investigate the hotel, her mind preoccupied with the earlier conversation she'd had with Han. She couldn't shake off the eerie coincidence of the glasses appearing precisely where she worked. Jun Seon, sensing something amiss, followed her inside, his concern evident. Hyun Heup attempted to elucidate the strange visions she experienced above people's heads, but her explanation only seemed to alarm Jun Seon further. In a moment of shared vulnerability and unexpected intimacy, they found themselves locking lips instead.
At Han-yu's apartment building, the detectives meticulously comb through the contents of clothing donation bins, stumbling upon what appear to be the very sheets from Han-yu's room. Summoned back for interrogation, the detectives are perplexed to find Ji Na's bloodstains on the linens. Yet, Han-yu weaves an excuse, professing his lack of interest in her. But a breakthrough comes as Han reviews the CCTV footage, spotting a crucial clue.
Strolling down the hallway, Hyun Heup remarks that four men all seem to have had connections or intentions towards Ji Na. She promptly dials Han's number, only to discover he's already at school. Rushing over in the squad car to the police station, Han pinpoints the weakest link among the suspects, and they whisk him away to the crime scene, patiently waiting for his resolve to crumble. It doesn't take long before he cracks.
Soon after, in separate confessions, the suspects all point the finger at Han-yu, accusing him of orchestrating the assault and drugging Ji Na. He vehemently denies being the actual rapist, but Han labels him as the mastermind behind the crime. As they drive home, with Hyun Heup by his side, Han remarks that the case must now be left to the judgment of the court. She wonders aloud if he feels a sense of emptiness, given all the "S Lines"—his intricate web of suspects and clues—he's had to untangle.
Their conversation is abruptly cut short by a call from Han's father's care home; his father has gone missing once again. Han hastily drops Hyun Heup off at the bus stop and races to the care facility. Reviewing the hospital's CCTV footage, they observe a woman ushering his father into a taxi. Could it be Kyu Jin? Han's mind races as he speeds toward Seon-A's hospital room, simultaneously instructing Dong Sik to delve into Kyu Jin's background.
Upon entering Seon-A's room, Han's eyes are drawn to her "S Line"—a term he uses to describe the subtle, telling curves of her body language—as she bends toward the door. To his utter shock, his father walks in, leaving Han stunned and bewildered. "How could this possibly happen?" he wonders aloud.
The scene then shifts to Hyun Heup, who is now drenched in blood, frantically calling Han for urgent assistance.
