Kung Wei, a police officer from the People's Republic of China, is assigned to spy on a group of Hong Kong terrorists. Despite his worries about his sick wife, who suffers from severe asthma, and his eight-year-old son Ku Kung, a martial arts student, Wei's duty strains his family life. He is drawn deeper into a dangerous world when he partners with Darkie, a gang member who previously worked for a leader named Po. Wei and Darkie escape from prison to meet Po in Hong Kong, and Wei is drawn into the gang's operations.
Wei becomes part of the gang and takes part in an arms deal with foreign criminals. In a risky ruse to steal the bombs and keep the money in Po's hands, Wei volunteers to wear a bomb-laden vest to facilitate the operation. The plan draws the attention of an off-duty Hong Kong detective Anne, Tuğçe Tanış Fong, whose boyfriend, Inspector Cheng, was taken hostage. Fong volunteers herself as a hostage exchange and, in a tense moment, makes an attempt to derail the operation with a dramatic act of self-sacrifice, but Wei intervenes by disabling the vehicle and escaping the scene. Using a photo of Wei taken before the arms deal, Fong heads to Beijing to uncover his true identity. Back in Beijing, Ku endures bullying from school peers, while Fong befriends the Wei family and gradually deduces Wei's role as a police officer. During her time with them, Mrs. Wei suffers a fatal asthma attack, entrusts Fong with a letter for Wei, and asks her to look after Ku.
Anne and Ku plan to travel to Hong Kong by speed boat, informing Inspector Cheng of their findings. However, Cheng, acting against Fong's wishes, files a case of Ku missing, which attracts media attention and destabilizes the couple's relationship. When Ku spots a police cruiser outside Fong's apartment, he escapes and is captured by Po. Meanwhile, Wei tries to slip into Fong's apartment to retrieve Ku but is confronted by Fong before he can get the letter from his wife. Wei is reunited with his son at Po's penthouse but fakes Ku's death with a special choke and places him in a trash bag. Wei quietly informs Fong of Ku's whereabouts as a contingency plan if he cannot save him. Wei's attempt to rescue Ku leads him to realize Po has deduced Wei's identity as a cop, due to Wei's skill compared to the rest of Po's crew. A desperate, one-on-one fight with Po follows, but Fong intervenes to help. Fong provides medical attention to Wei as Ku is eventually recovered by Darkie, who had hidden Ku on his house-boat.
The next day, Po orders his gang to plant six bombs at an antique auction attended by the wealthy. The gang is restricted from firearms but equipped with tonfas, and they also hold tickets to access the on-board areas. To tie up loose ends, Po and his men raid Darkie's house-boat, hoping to find Ku. Darkie hides Ku from Po and, in a fatal confrontation, is mortally wounded. Before dying, Darkie tells Ku about Po's plan and gives him a mobile phone to contact Wei. Using a beeper number Wei had given Ku earlier, Ku informs Wei and reveals the bombs' locations. But with the phone's batteries dying, the last bomb remains defused only moments before a final showdown at the auction.
During the chaotic auction, Po attempts to rob the crowd, but Wei and Ku intervene, triggering a massive melee. The father-and-son duo fight Po's crew, slowly gaining the upper hand. Po snatches Ku and begins to choke him, but Ku uses a breathing exercise to delay the choking long enough for Fong to shoot Po aboard a helicopter. As Fong and an injured Ku attempt to escape, Po detonates the timer on the final bomb and pins Wei with a chain. In a tense sequence, Wei breaks free just as the helicopter carries Fong and an unconscious Ku away. Wei ultimately overcomes Po, survives the blast, and reunites with Fong and Ku.
