The fourth episode of "A Killer Paradox" unfolds with a group of colleagues enjoying a camaraderie-filled dinner, their laughter resonating through the restaurant. Prosecutor Ji, their boss, hands out his business cards before excusing himself to head home, summoning a driver whom he mistakenly assumes is already awaiting him beside his vehicle. The figure standing there, cloaked in a hoodie emblazoned with "Only For Heroes," blends into the night. Halfway through the journey, Ji is jolted awake by a phone call from the actual driver, realizing his mistake too late. Suddenly, a flurry of flashing lights pierces the car's interior, and the scene cuts to a grim discovery: a body drifting aimlessly down a river.
Elsewhere, Yeong makes his way to visit the convenience store owner, who unexpectedly withdraws his complaint against Tang. The revelation that Tang's family had settled the debt owes explains the change of heart. Meanwhile, Chung-jin bids farewell to his days at the police station, marking the inception of a special investigations unit dedicated solely to solving the series of murders. Four months pass, yet the unit remains empty-handed, prompting Nan-gam to sneer that the entire endeavor was a charade. His scathing remark doesn't escape the ears of the unit's female leader, sparking a heated debate. Nan-gam is convinced that a singular perpetrator is behind all the killings, while she remains skeptical.
In a room filled to the brim with superhero memorabilia, a young man named Roh Bin engages in an online discussion about a murder case, his insights hinting at a profiler's acumen. Nan-gam pays him a visit, laying out the details of multiple murder cases. Bin ponders aloud the possibility of a serial killer's involvement. Unbeknownst to Bin, Nan-gam has a colleague eavesdropping on their conversation. When Bin casually mentions hacking into the prosecutor's network, Nan-gam signals his colleague to escort Bin to the station.
At the station, Nan-gam persuasively argues his case to let him handle Bin, believing the latter holds vital information. Despite lacking concrete evidence, they eventually release Roh Bin, a move that seems calculated within Nan-gam's broader strategy. During the ride back to Bin's place, Nan-gam vents his frustration over the lack of leads and urges Bin to share any insights he might uncover. Later, while munching on snacks at the convenience store, Nan-gam's eyes catch sight of Rex, Yeo-ok's dog. Curiosity piqued, Nan-gam trails Rex to Yeo-ok's residence, setting the stage for yet another twist in this intricate web of murder and mystery.
A visitor arrives at Roh Bin's residence with a food delivery, revealing himself to be Lee Tang. They exchange signals hinting at a collaboration, prompting Bin to caution Tang to exercise caution. Meanwhile, outside, Nan-gam's colleague intercepts Tang and inquiries about the apartment he is delivering to. In a nearby vehicle, Nan-gam informs Yong-jae that Roh Bin once served under the murderer Nan-gam was trailing. Unbeknownst to them, Tang is engaged in conversation with the other officer. The colleague then joins Nan-gam in the car, relaying that the 'delivery man' claims he has never laid eyes on Roh Bin's face. Since Nan-gam personally witnessed Bin opening the door to receive the delivery, he recognizes the statement as a fabrication. He instructs the officers to pursue the delivery man forthwith.
Yong-jae and another officer set off to chase Lee Tang, while Nan-gam storms into Roh Bin's apartment. He confronts Bin, demanding to know if he has been selecting victims for Tang and urges him to disclose Tang's whereabouts. Yong-jae returns with the delivery man, but it transpires that they have the wrong individual. The captain then calls Nan-gam, scolding him sternly.
It emerges that Roh Bin has installed cameras in his apartment, and videos depicting Nan-gam assaulting him have been disseminated across the internet. The captain is incandescent with rage. Additional policemen arrive at Bin's doorstep.
A flashback unfolds to four months prior. Tang packs his belongings and bids farewell, leaving a letter for his family announcing his sudden departure to Canada without a proper goodbye. He vows to return as a better version of himself but finds himself slumbering at a railway station. He searches for 'Only For Heroes' online but comes up empty-handed. His days blur into a routine of riding trains until one day Roh Bin approaches him, introducing himself as 'Only For Heroes.'
Roh Bin escorts Tang back to his place and proposes a collaboration, positioning himself as a trusty sidekick akin to Watson or Robin, a moniker he legally adopted as Roh Bin. He reveals a startling knowledge of Tang's dark deeds, including the intriguing superpower that makes Tang's victims seemingly deserving of their fate, with all incriminating evidence mysteriously vanishing. Roh Bin tempts Tang with the promise of wealth, suggesting they could embark on a quest to uphold justice together.
The following dawn, Roh Bin and Tang delve deeper into the latter's extraordinary abilities. Bin mentions that Tang has already crossed paths with one of his two former partners, hinting that the other is someone Tang would prefer to avoid. The first partner is revealed to be Bu-il. Bin directs Tang to a secret compartment behind a shelf, revealing an arsenal filled with weaponry, documents, and a jacket emblazoned with "Only For Heroes." Tang slips into the jacket and strides confidently through the streets until he unexpectedly encounters Prosecutor Ji. Instinctively feeling the urge to eliminate him, Tang pretends to be Ji's chauffeur, later incapacitating and interrogating the prosecutor to uncover his sins.
Roh Bin arrives on the scene and hacks into Ji's phone, uncovering videos that expose the prosecutor's sexual assaults on multiple women. For Tang, this is undeniable proof of Ji's guilt, and he stabs Ji without hesitation. Together, they watch his lifeless body drift away. Tang's confidence grows as he continues to eliminate those he deems worthy of punishment, even enhancing his physical prowess to overpower adversaries easily.
Meanwhile, Nan-gam and his entire team face severe reprimand. His captain gets involved in a physical altercation, leading to both their dismissal. At the Chinese restaurant where Tang works, a cop named Song Chon inquiries about Lee Tang at the conclusion of A Killer Paradox Episode 4, setting the stage for a potentially perilous encounter in the future.
