"阿Sir" is a way to address both teachers and policemen. The meaning of the title is "So it turns out '阿Sir'(teacher) was an '阿Sir'(policeman)".
Plot
Despite the fact that he comes from a long line of disciplined workers, Law Yiu-wah (Moses Chan) is a teacher of a notorious school. One day, he was urged to resign after being vilified by his students as a peeping Tom.
At present, the triads have expanded their drug network at local schools. Wah, who has turned into a cop, is told to go undercover at the school where he used to teach.
At school, he is impressed by the enthusiasm of a new teacher Ho Miu-suet (Tavia Yeung). He also comes to realize he was too strict with his students in the past. With a determination to boost students’ confidence, Suet invites bowling expert Koo Ka-lam (Linda Chung) to teach bowling at the school.
When Wah learns about her triad background, he decides to go after her hoping to learn more about the triads. Wah also finds himself in a contest of wits with Inspector Ching Man-lik (Ron Ng), who has been assigned to work as a liaison officer at the school.
On one hand, Man is attracted to Suet but on the other, she only loves Wah. All of a sudden, Wah, Suet, Lam and Lik find themselves involved in a complicated love square.
