The second of three urban action thrillers Yuen Woo-Ping and Donnie Yen collaborated on as director and star, In The Line Of Duty 4 is also, you guessed it, the fourth installment in a franchise that only has vague thematic continuity between its installments. The first two In The Line Of Duty films starred Michelle Yeoh and are also known as Yes Madam! and Royal Warriors. For the third film, Yeoh pulled out and was replaced with Cynthia Khan, who introduced the character of Rachel Yeung, which she reprises in this fourth film.
Cynthia Khan emerged as a replacement for Michelle Yeoh in the series and in Hong Kong cinema in general, after Yeoh went on an early and temporary retirement at the end of the eighties. She is just as beautiful and has the same tomboyish style as eighties Michelle Yeoh, but the difference is she is often replaced with an obvious stunt double in the trickier action scenes. Still, she is a charismatic and charming presence, and it’s a pity she vanished from the mainstream in the mid-nineties. Here she is paired with Donnie Yen (then at the beginning of his career and still a protégé of director Yuen Woo-Ping) and they play two cops investigating a drug-trafficking network with possibles international ties. They wind up having to take care of an innocent dock worker (Yuen Yat Chor) who witnessed a murder that is pivotal to the case, and questioning the loyalty of colleague Wong (Michael Wong), who might be playing both side.