Chow Sing-sing (Stephen Chow) is about to be disqualified from the Royal Hong Kong Police's elite Special Duties Unit (SDU) because of his complete disregard for his teammates during a training drill. However, a senior officer, Wong Sir (Barry Wong), who has taken to Sing's youthful demeanor, instead deploys him as an undercover student in the elite Edinburgh College to recover his stolen revolver. The undercover operation is made complicated when Sing is partnered with Tat (Ng Man-tat)—an ageing, incompetent police detective. Sing, who turned to the Police Academy because of his dislike for schooling, struggles to fit in academically. On just his third day at the school, Sing is on the verge of being expelled due to being caught cheating on a Chinese history test. Tat, who is listed on files as Sing's legal guardian, beats him ruthlessly after much prodding from the school's administrative staff. Miss Ho (Sharla Cheung), the school's guidance counselor, volunteers to personally tutor him.
The school bully, Johnny (Leung Sap-yat), challenges Sing to a fight on the school rooftop. Turtle Wong (Gabriel Wong), Sing's classmate who had also cheated on the Chinese history test but had been protected by Sing taking the blame, is the only one to take his side. Sing wins easily, earning the respect of his peers. With Miss Ho's help, Sing becomes a model student, in part due to his growing infatuation for her. This relationship puts her at further odds with her boyfriend, Detective Wong (Wong Chi-yeung), another fellow officer who is also investigating the arms traffickers suspected of stealing Wong's pistol. Already distasteful of police officers, Miss Ho is dismayed when she discovers both Sing and Tat's real identities and distances herself from the two. Later, she finds her boyfriend ruthlessly beating Sing, causing their relationship to end permanently.
After two months pass with little progress, Tat and Sing finally decide to pursue the arms traffickers and their boss, Teddy (Roy Cheung), who is also Johnny's older brother. Sing manages to escape from the lot with a truckload of weapons and returns Wong's stolen pistol, but accidently leaves Tat behind with the criminals, who hold him captive. Sing rushes back to rescue him and the two flee back to the school with a suitcase containing the traffickers' money. The school is setting up a maze for seasonal festivities and Tat and Sing meander through it to escape their pursuers. Aided with the help of some of the students, the two are able to ward off their attackers, but just as Sing, Turtle, and Johnny rendezvous, the remaining students are captured at gunpoint by Teddy and his men. The trio are given five minutes to surrender, or the students will be executed. Sing overpowers Teddy's crew after Turtle and Johnny distract them, but Teddy then holds Miss Ho at gunpoint. Wong arrives in time to subdue and arrest Teddy with Sing's help. As Wong takes his leave, he spontaneously promotes Sing to Chief Inspector.
Sing says his final goodbyes to his classmates as his mission is complete. But as he turns to leave through the school's main gates, he and everyone else find Miss Ho seductively waiting for him.
