The gripping premiere of "Mercy for None" unfolds in Yeouido, Seoul, amidst the bustling year of 2010. A figure named Gi-jun shares with his youthful sibling, Gi-seok, the pivotal news of his conversation with the esteemed Chairman Lee Ju-woon. With a mere mention, Gi-seok's future is brightened with the prospect of soon joining the company's fold.
As dawn breaks on Gi-jun's intentions, he strides purposefully towards a turbulent confrontation between two rival gangs. In a display of unparalleled prowess, he effortlessly fells a formidable adversary from the opposing side, who had previously overpowered several opponents with sheer brute force.
The narrative then pivots to the present day, where a manager from the prestigious Bongsan Group finds himself entangled in a perilous predicament, kidnapped by two ruthless foreign assassins who dispatch him with clinical precision.
Chairman Gu Bong-san of Bongsan Group and Chairman Lee Ju-woon of Juwoon Group converge over a meal to dissect this alarming incident. Bong-san recounts how his reckless son, Gu Jun-mo, in a fit of panic over uncovering business irregularities, orchestrated the hiring of these foreign killers to eliminate his manager. Cognizant of the delicate family ties involved, Chairman Bong-san hesitates to intervene directly, instead pleading with Ju-woon to handle the menacing assassins and restore order. In exchange, he promises assistance in resolving a thorny real estate dilemma.
Chairman Ju-woon, after a moment of contemplation, assents to the request, entrusting the daunting task to Gi-seok, now a senior managing director and the designated heir apparent to Ju-woon's legacy. Yet, beneath the veneer of duty, Gi-seok harbors a desire to sever ties with his professional life. Despite this internal conflict, he embraces the challenge, embarking on a visit to Jun-mo alongside his loyal junior, Cheon Hae-beom, as the intricate web of intrigue tightens its grip.
Gi-seok issued an ultimatum to the foreign assassins, commanding them to vacate their premises within three days. Jun-mo, dissatisfied with this decision, taunted Gi-seok by revisiting the incident involving Gi-jun. Apparently, Gi-jun had caused quite a stir, which both chairmen had to meticulously rectify. As punishment, they spared his life but inflicted a cruel penalty by severing his Achilles tendon. With swift and decisive blows, Gi-seok subdued Jun-mo, who continued to hurl death threats at him as he walked away.
Elsewhere, Kim Chun-seok, a manager at Bongsan, received a call that left him astonished—Gi-seok had personally confronted Jun-mo.
Subsequently, Gi-seok ventured into the woods to reconnect with Gi-jun, who had been living a secluded life at a campsite for six years. Upon meeting, Gi-seok imparted that Chairman Ju-woon wished for Gi-jun to become the successor, yet he harbored mixed emotions about it. Gi-jun, sensing something amiss, grew inquisitive.
Returning home, Gi-seok felt an eerie presence in the house. Descending to the parking lot, he discovered a cat. As he fed the feline, a masked assailant struck from behind. Shadows came alive with more attackers—an obvious ambush. Despite the odds, Gi-seok dispatched them all with ease. He spared the last man, but treacherous deceit awaited him; as his guard dropped, the man lunged and stabbed Gi-seok.
Meanwhile, deep in the woods, Gi-jun received a phone call, and the narrative abruptly shifted to Gi-seok's funeral. Employees of Juwoon, Chairman Gu Bong-san, and Jun-mo paid their respects. Outside, Choi Seong-cheol, Ju-woon's manager, whispered suspicions to Kim Chun-seok, implying Bongsan's involvement in the attack. Chun-seok dismissed it as mere mischief by runaway children, citing police verification.
At that precise moment, Prosecutor Lee Geum-son, the eldest son of Lee Ju-woon, arrives to pay his condolences. However, Seong-cheol promptly requests his departure, explaining that his father would prefer his absence. Shortly thereafter, Gi-jun enters the scene, prompting Seong-cheol to command everyone to bow to him. Leading Gi-jun to Gi-seok's lifeless body, he inquires whether the perpetrators were indeed mere children.
Elsewhere, the two chairmen deliberate on the succession of Ju-woon's position. Gi-jun bursts into the room, reminding them of a pact made eleven years ago. He had vowed never to resurface, while they had pledged to safeguard his brother's well-being. When they offer no further insights into his brother's demise, Gi-jun storms out.
Outside, Hae-beom introduces himself to Gi-jun, who subsequently heads to Gi-seok's abode. There, he discovers an assortment of camping books and a birthday gift intended for him, thoughtfully prepared by Gi-seok. Emotionally moved, Gi-jun wraps a bandage around his ankle, the one bearing a scar from their shared past.
Determined, Gi-jun proceeds to the police station, eager to confront the suspect in his brother's murder case. Though initially denied, he subtly hints at possessing inside knowledge about one of the detectives, securing a meeting.
The suspect, Hui-chan, displays no hint of contrition but inadvertently mentions having a brother himself. Sensing an opportunity, Gi-jun pursues Hui-chan's younger sibling and finds himself cornered by Hui-cheon's entire gang. This confrontation was, in fact, Gi-jun's strategic maneuver to uncover Gi-seok's killers. The brawl ensues, with Gi-jun effortlessly dispatching his adversaries. Once all are subdued, he advances toward the younger brother.
In a poignant epilogue to Mercy for None Episode 1, we witness Hui-chan stabbing Gi-seok, who nonchalantly dismisses the attack and watches as the gang flees. He extracts the knife from his side and makes his way out of the parking lot, only to find someone awaiting him on the other side of the door, leaving the fate hanging in suspense.
