The premiere of "The Nice Guy" unfolds with our protagonist, Seok-cheol, returning from a bout of fisticuffs, eager to meet Mi-young at the bustling bus station, only to be abruptly plunged into a harrowing turn of events when he is stabbed. The narrative then transitions to Seok-cheol reminiscing about his bygone aspirations of becoming a poet or novelist, as we witness him amidst a poetry workshop where his true identity as a gangster is unexpectedly revealed to the assembled crowd.
Meanwhile, Seok-cheol receives startling news: his police officer friend is orchestrating a mediation session between two rival gangs, one of which he himself belongs to. This mediation is being overseen by none other than his father, a retired gangster with a colorful past. Upon returning home, Seok-cheol learns of a familial crisis—his eldest sister, Seok-kyung, has gambling debts so colossal that she lost the deed to their ancestral home and has gone into hiding, abandoning her son to the care of their doting grandparents. Furthermore, his youngest sister, Seok-hee, a dedicated nurse, is in a blossoming relationship with one of Seok-cheol's former schoolmates, now a distinguished doctor.
On his mission to deliver a care package of kimchi to Seok-hee at her workplace, Seok-cheol detours to his "workplace" to inform his gang boss of his intentions to quit the treacherous life of a gangster. After some negotiation, they strike a deal: he shall be granted his freedom...provided he undertakes one final task—reclaiming land occupied by protesters who are vigorously resisting a redevelopment project.
As Seok-cheol awaits his sister's arrival, fate conspires to reunite him with Mi-young, a musician with a YouTube channel where she shares her heartfelt compositions. Recently, she had turned down a performance offer due to crippling stage fright. Their shared history becomes evident, and they soon discover a surprising connection: Mi-young is actually friends with Seok-hee.
Later on, we uncover that during their school days, Seok-cheol and Mi-young harbored mutual affection until Mi-young's abrupt disappearance. Through Seok-hee, we further discover that Mi-young has been grappling with financial woes, prompting her to seek assistance from Seok-hee.
The following day, Mi-young arrives at an audition she had diligently prepared for, but her courage wanes as more people flood in. Unable to muster the courage to sing before a live audience, she forfeits her opportunity.
In the meantime, Mi-young and Seok-cheol reconnect and agree to meet once more. Mi-young confides in him about her stage fright, and Seok-cheol grants her the space to unfurl her emotions. They pay a visit to Mi-young's mother at a nursing home, where Seok-cheol urges her to serenade the elderly residents. Despite her reluctance and haunting memories of bullying, she manages to deliver a brief, heartfelt performance. Subsequently, Seok-cheol escorts her back home.
That very evening, Seok-hee reaches out to Seok-cheol with the news that she has traced Seok-kyung. One of Seok-kyung's closest friends had visited the hospital for a herniated disc, and through her, Seok-hee unearths Seok-kyung's whereabouts.
The siblings hurry to a gambling den where Seok-kyung has been employed. Mayhem ensues as the den's owner and her minions assault Seok-cheol and Seok-hee when they attempt to reclaim Seok-kyung. Amidst the tumult, Seok-kyung witnesses the owner attacking Seok-hee and, driven by a surge of anger, headbutts her, bringing the episode to a cliffhanger.
