In Macau, criminal Tong Heung-tung of the Hunan Gang, who once bombed the Hong Kong police headquarters, is being sent back to Hong Kong under escort by Kowloon OCTB inspectors Lee Ho-lung and Lo Mo. Mid‑transfer, the police van is ambushed by Tong’s gang, enabling his dramatic escape. [Lee Ho-lung] relentlessly pursues the fugitives across the Mainland border, eventually arresting them but leaving behind a trail of destruction that enrages his superior, Chan Sir.
Lau Kwok-wah, a diligent inspector from the Complaints Against Police Unit, is Lee’s steadfast ally. One night, while Lau, Lo Mo, Lee, and Lee’s wife and daughters share a meal, Miss Chung Miss Chung arrives seeking help. TheHunan Gang’s leader Wai engineers Chung’s use of Lee to settle old scores, using her to lure him out for revenge. When Lee reaches Chung’s house, he clashes with Wai’s henchmen and is overwhelmed, getting knocked out by Wai. Chung is also knocked unconscious and overhears the gang’s plans; she escapes, but is pursued, and she narrowly avoids being blamed for killing a henchman. Wai and his gang drag Lee to the pier, brutalize him, intoxicate him, and force him to drive himself into the ocean, resulting in Lee’s death.
The morning after Lee’s death, his wife May gives birth to a baby boy and mourns the loss of her husband. At the police station, Lau retrieves Lee’s belongings and voices suspicions about the circumstances surrounding Lee’s death to Chan, who remains skeptical. Lau and Lo Mo decide to investigate, starting with gathering information from Chung. They locate Chung at a nightclub, where narcotics officers harass her until Lau intervenes, and Lau and Lo Mo take Chung to a restaurant only for her to slip away. She flees to her grandmother’s house, where the Hunan Gang attempts to kill them by setting the house on fire; Lau and Lo Mo rescue them and bring Chung to the police station, where she tells Chan that Wai and his gang abducted Lee, a claim Chan still refuses to accept. Frustrated by Chan’s inefficiency, Lau resolves to pursue the case on his own.
Determined, Lau invites Chung to stay with him and, in return, Chung agrees to help locate Squid Chiu, the leader of the Hunan Gang, whom Lau jokingly threatens to use as bait. Chung guides Lau toward Al, a low-life Triad member who might help secure Chiu’s cooperation. When Al balks, Lau reveals he has used Al’s name to contact Chiu and schemes to frame Al for drug possession or accuse him of betraying Chiu. Reluctantly, Al agrees, convinced by Lau’s leverage. Chiu agrees to see Lau only after Lau manipulates a plan to have him rob Lau’s jewelry store.
To execute the ruse, Lau asks Lo Mo to borrow his future father‑in‑law’s store for a storefront—an arrangement dependent on a willing date for Lo Mo’s girlfriend’s father, who would be absent. Chung, who is battling drug withdrawal, carries two heroin-laced cigarettes in her purse; Lau discreetly discards them in the toilet to prevent trouble, and a traffic jam creates a crucial delay that allows them to slip into position just in time. When Chung’s addiction flares during the staged encounter, Lo Mo quickly supplies methadone, and together they proceed with the plan involving Lo Mo’s father-in-law and Chiu.
Back at home, Lau scolds Chung for nearly derailing the operation while Chung blames him for discarding her cigarettes. She confides that quitting is not easy without support, and Lau consoles her with a promise: after avenging Lee, he will take her to a quiet place on Lantau Island and help her quit.
The following day, Lau wires HK$200,000 to Chiu, but Lo Mo informs Chan, who refuses to participate in the robbery. Chung then hints that a Taiwanese drug dealer will arrive in Hong Kong the next day, prompting Lau to devise a plan to rob the dealer. Lo Mo initially hesitates, but Lau reminds him that Lee treated him as a close friend and urges him to join for Lee’s sake, and Lo Mo finally agrees.
At the airport, Lau, masked and disguised, abducts the drug dealer as Lo Mo and Chung arrive in a van to collect him, all while being pursued by the police. Lau forcibly removes the dealer from the car, and Lo Mo inadvertently reveals their status as police officers. They seize the money and escape as shots ring out.
Back at OCTB headquarters, Chan learns that Interpol Agent Chan Kwok-chiu, who had been undercover as the Taiwanese dealer, was robbed, and suspects police involvement. The Interpol agent describes a 200-pound suspect, prompting Lo Mo to attempt a drastic weight loss ruse that fails. A plan to alter his Cantonese accent is tried; Lo Mo pretends to speak with a Chiuchow accent, but the ruse is exposed.
Lau eventually delivers the HK$200,000 to Chiu. That night, while Lau types a report, Chung appears and Lau offers her money to go to Lantau Island if the investigation drags on; she refuses, and the two share a drink and a kiss in a quiet moment.
The next day, Lau escorts Chung to the harbor to send her away, and Lo Mo drives him to the jewelry store where they will monitor the robbers. Lau instructs Lo Mo to alert Chan about the robbery by 12:30 PM. Lo Mo attempts to raise the alarm, but Chan is in a meeting. Interpol Superintendent Lai orders Lo Mo’s arrest, while Lau and his ally face the oncoming confrontation. At the jewelry store, Wai’s gang arrives, and Chung reveals that Wai captured Lee on the night of his death. The Hunan Gang robs the store, but the police have not yet arrived. Lau and the beat officers call for backup, triggering a dramatic chase and gunfight. Lau eliminates several gang members, and Wai retaliates by killing one of the beat cops. The pursuit leads them to the roof of a building, where bullets run dry and each side struggles for the dead police officer’s gun. Wai gains the upper hand, but Chung arrives with a gun aimed at Wai. Lau, and the surviving beat cop, fight to survive; Lau finally disables Wai, shooting him repeatedly after a tense struggle. As the gunfight ends, Chung lies dying in Lau’s arms, telling him that no one cared for her as deeply as he did and that she could not quit drugs without him. She dies, and in the aftermath, Lau and Lo Mo are arrested in connection with the airport robbery of Chan Kwok-chiu.
