Sun-woo, Lee Byung-hun, is a high-ranking enforcer and loyal right-hand to crime boss Kang, Kim Yeong-cheol. The two share concerns over tensions with Baek Dae-sik, Hwang Jung-min, a rival family’s son. Recently Sun-woo beat up Baek’s men for overstaying their welcome at their nightclub, signaling the escalating pressure between their circles. As Kang prepares to leave on a business trip, he assigns Sun-woo to shadow his young mistress Hee-soo, Shin Min-a, whom he fears is having an affair with another man.
As Sun-woo carries out his duty—following Hee-soo, and escorting her to a music recital—he becomes quietly enthralled by the girl’s beauty, while glimpses of his lonely, empty personal life begin to surface. When he finally discovers Hee-soo and her lover at her home, he beats the man and considers informing Kang. Yet he changes his mind, sparing them on the condition that they never see each other again, a decision that earns Hee-soo’s enmity and unsettles the fragile balance of power around him.
Later, a man asks Sun-woo to apologize for beating Baek’s men, but he refuses. Agitated, he drinks alone in his apartment and is kidnapped by Baek’s henchmen. They are ready to kill him, but Kang intervenes with a phone call. Kang has learned of Sun-woo’s attempted cover-up and questions his motive, but does not press the issue. He orders Sun-woo tortured, yet gives him a chance to repair the damage. Sun-woo escapes and doubles down on his vow of revenge.
Sun-woo sends Hee-soo a farewell gift and then tries to buy a handgun. The deal goes sour, and he ends up killing the arms dealers instead. This act triggers a vendetta from the arms dealer’s brother, who confronts Sun-woo at the nightclub where Hee-soo works. Sun-woo lures Baek to an ice rink and kills him, though he sustains injuries in the struggle.
Undeterred, he fights his way into the nightclub and faces Kang, blasting away at years of loyalty and perceived betrayal. When Kang offers no justification, Sun-woo shoots him as well. Baek’s henchmen, trailing the pair, exchange fire with Kang’s men, leaving Sun-woo as the lone survivor of the melee. Just as the arms dealer’s brother appears, Sun-woo bleeds out, and memories of Hee-soo’s recital flash across his mind—the one moment that made him smile. The brother then delivers the final strike.
The film closes with a return to an earlier moment, as Sun-woo gazes out over the city from a window. Alone, he begins to shadowbox his reflection in the glass, a quiet, solitary happiness returning to him in the aftermath of everything he has done.
