Wah [Andy Lau] is a mob enforcer in Mong Kok who primarily handles debt collection, moving between the neon-lit alleys and backstreets that define the district, while his hotheaded and overambitious protégé, Fly [Jacky Cheung], struggles to find real success and often drags him into trouble. One day, Wah receives a call from his aunt informing him that his younger cousin Ngor, [Maggie Cheung], who works at her family's restaurant on Lantau Island, is visiting Mong Kok for a doctor's appointment and will be staying with him briefly.
Soon after Ngor arrives, Wah leaves to help Fly collect a debt. After the job, Wah visits his girlfriend, Mabel [Wong Aau], who dumps him, revealing that she aborted his baby. Wah returns home drunk and furious, and when Ngor tries to console him, his temper flares and he threatens to throw her out. The next morning, Wah apologizes and tries to make amends, offering to take her out to a movie to smooth things over.
To pay off a loan from Tony, a fellow gangster, Fly makes an unreasonably high bet in a game of snooker. Realizing there is no chance he can win, he bolts with his friend Site through the streets. Tony's gang gives chase, and the two are brutally beaten. Wah and Ngor are about to leave for the cinema when Fly arrives, bruised and carrying an unconscious Site. They tend to the wounds, and Ngor starts to question Wah's line of work. The next day, Ngor returns to Lantau, leaving Wah a note asking him to visit someday.
Wah goes to Tony to settle Fly's debt. Unable to pay the interest, Wah robs the establishment that Tony is hired to protect. Tony feels insulted and takes his grievance to Uncle Kwan, the mob boss, who forces a compromise. Kwan tells Wah to get Fly under control before the latter winds up dead. Conceding that Fly is not cut out for mob life, Wah arranges a legitimate job for him—selling fishballs from a small cart.
Wah later runs into Mabel again and discovers she is now married. He makes a quiet, stubborn visit to Ngor on Lantau, waiting a full day to meet her at the ferry terminal. Over the following days, Wah helps out at the family restaurant and grows closer to Ngor, and their bond deepens as he begins to see a possible future beyond the violence. Meanwhile Tony insults Fly at his new job, and Fly answers with a reckless act that wrecks Tony's car; a swift counterattack from Tony's gang leaves Fly nearly dead. Wah receives a call from Tony demanding money in exchange for Fly's life, and he races to Tony's place.
Wah bursts in, surprises Tony, and forces his release of Fly at gunpoint. He urges Fly to return to his family in honest work, but Fly refuses, clinging to a misguided sense of loyalty and bravado. As they step out of a bar, they are set upon by Tony's men and brutally beaten. Fly begs Wah to leave him behind, admitting he is a failure and a terrible friend. Wah staggers back toward Lantau, where Ngor and her doctor treat his wounds. He slowly recovers, and the two rekindle their relationship, clinging to the fragile hope of a quieter life together.
Back in Mong Kok, Uncle Kwan arranges a new assignment for Tony—an assassination of an informant. Tony is reluctant, a mission that looks to be a suicide, but Fly volunteers, eager to prove himself. Fly humiliates Tony in front of the others, and the mob’s loyalty begins to fray as Tony’s thugs abandon him in disgust. Hearing this, Wah tracks Fly down, trying to talk him out of taking the hit. At the police station, Fly shoots at the informant but misses; he is quickly gunned down by the police. Wah arrives in the nick of time, seizes Fly’s gun, and finishes the job himself, only to fall under a hail of bullets as well. In his final moments, he remembers that first kiss with Ngor, a memory that stands in stark contrast to the violence that defined his life.
