The Royal Hong Kong Police Force is planning a major undercover sting called "Operation Boar Hunt" to arrest crime lord Chu Tao (Yuen Chor).
Inspector Chan Ka-Kui (or Kevin Chan in some versions) is part of the operation, along with undercover officers stationed in a shanty town. However, the criminals spot the police and the ensuing car chase cuts through the hillside shanty town, vehicles destroying the shacks and causing large explosions.
Ka-Kui persists in his chase, eventually following on foot as the drug lord attempts to escape in a double-decker bus. Ka Kui catches the bus and initially is able to hang on to it using an umbrella, but is thrown off. He then manages to get in front of the bus and bring it to a halt by threatening to shoot the driver with his service revolver. Later, Ka-Kui is reprimanded by Superintendent Li for letting the operation get out of hand, but is subsequently presented to the media as a model police officer.
His next assignment is to protect Chu Tao's secretary, Selina Fong (Brigitte Lin), who plans to testify in court about Chu Tao's illegal activities. At first, Selina insists that she does not require protection, but after Ka Kui has a fellow policeman break into her apartment and pose as a knife-wielding murderer, she becomes more cooperative. After Ka-Kui and Selina leave her apartment later that evening, they are attacked by some street thugs, whom Ka-Kui is able to defeat with his martial arts skills, though the fight leaves Selina's car a wreck.
When Ka-Kui arrives at his apartment with Selina, who is only wearing lingerie, he is surprised by his girlfriend, May (Maggie Cheung) and her friends, who are throwing a birthday party for him. May, seeing the scantily-clad Selina, misunderstands and becomes angry with Ka-Kui, shoving the birthday cake into his face. Ka-Kui is later able to explain to May that Selina is a witness, but only after much bumbling and further misunderstanding.
Meanwhile, Selina has discovered that the attack by the man with knife at her apartment was a sham, and so she decides to not cooperate with Ka Kui. She sneaks away while Ka-Kui is sleeping and is not present for the crucial court date the following day.
Though Chu Tao is released on bail, he wants revenge against Ka-Kui. Using a corrupt policeman, Inspector Man (Kam Hing Ying), Chu Tao is able to frame Ka-Kui for the murder of Inspector Man. Now a fugitive cop killer, Ka Kui must try to catch Chu Tao and clear his name.
The action comes to a head in a shopping mall, where Chu Tao has an office. After surviving a murder attempt by Chu Tao's men to ensure her silence, Selina goes to the office to download incriminating data from Chu Tao's computer system. Chu Tao notices that the data is being dumped, and he and his men head to the shopping mall to intervene. Ka-Kui, who's monitoring Chu Tao's activities, follows. In the ensuing fight, Ka-Kui defeats all of Chu Tao's henchmen. The briefcase containing the computer data falls to the ground floor of the mall, but Chu Tao retrieves it. Ka-Kui, at the top floor, leaps off a ledge and grabs a pole wrapped in lightbulbs. He rapidly slides down the pole, smashing through the bulbs, crashing through a glass ceiling, and finally reaching the floor, where he violently apprehends Chu Tao but is held back humorously by his two friends Tak and Kim to stop him from delivering one final kick to Chu Tao.