Chow Sing-sing is on the verge of being disqualified from the Royal Hong Kong Police's elite Special Duties Unit after a training drill reveals his complete disregard for his teammates. A senior officer, Wong Sir, who has grown fond of Sing's youthful energy, decides to use him in undercover work instead—posing as a student at the Edinburgh College to recover a stolen revolver. The mission pairs him with Tat, an aging, often bumbling detective who is listed on files as Sing's legal guardian, which only adds to the tension of their uneasy partnership. Sing, who joined the Police Academy because he dislikes formal schooling, struggles to keep up academically and on just his third day is nearly expelled for cheating on a Chinese history test. The school staff pressure Tat to discipline him, and he beats Sing ruthlessly, a moment that complicates Sing's already fragile standing at the academy.
Miss Ho, the school's guidance counselor, volunteers to tutor him personally, hoping to steer him toward a better path. The school bully, Johnny, challenges Sing to a rooftop fight, and Turtle Wong, Sing's classmate who once took the blame for their shared cheating, is the only one who stands by him. Sing wins the confrontation, earning the respect of his peers, and with Miss Ho's guidance he gradually becomes a model student. His growing feelings for Miss Ho complicate things, as she is already involved with a fellow officer who is investigating arms traffickers, Detective Wong, and their professional and personal lines begin to blur. When Miss Ho discovers Sing and Tat's true identities, she distances herself, and her relationship with Detective Wong deteriorates after he learns Sing’s role in the case.
The investigation stalls for two months, pushing Sing and Tat to press forward against the arms traffickers and their boss, Teddy. Sing manages a daring escape with a truck full of weapons and returns Wong's pistol, but he leaves Tat behind with the criminals. The duo eventually flee back to the school with a suitcase full of money, while the festival environment—an elaborate maze being built for seasonal celebrations—becomes the backdrop for their escape. With help from some students, Sing and Tat fend off their attackers long enough to regroup, yet Teddy and his men capture the remaining students at gunpoint just as Sing, Turtle, and Johnny reconnect. They’re given five minutes to surrender; Sing overpowers Teddy's crew with distractions from Turtle and Johnny, but Teddy holds Miss Ho at gunpoint. Wong arrives to subdue and arrest Teddy, with Sing providing crucial assistance, and as Wong departs, Sing is promoted to Chief Inspector.
In the final farewell, Sing says goodbye to his classmates as the mission concludes, but as he walks toward the main gates, he finds Miss Ho waiting for him in a moment that feels both tense and charged, leaving the door open for what comes next.
