In the Line of Duty IV (1989)

Two detectives who are up to their necks in trouble and in each other's face, as they try to shut down a drug-trafficking scheme that could be connected with international ties to organized crime. But in the mist of their investigation, innocent immigrant dock worker Luk Wan-Ting gets caught up in the mix when he witnesses the murder of an intelligence operative and is framed for the crime.

After escaping police custody and after his friend is brutally killed by hired assassins in search of a film negative that Luk lost, he has no choice but to return to Hong Kong until the heat cools off. Donny and Penny Yeung are called in to extradite him with the aid of Captain Michael. Penny believes that Luk is innocent and tries to do everything in her power to help him, while Donny who is skeptical about Luk's innocence, thinks he's guilty and does his best to snap the cuffs on him. As the two do more investigating, they learn that the drug cartel could have connections with rogue C. I. A. operatives, who could also have someone on the police force keeping tabs on them about their investigation. At this point, Penny and Donny don't know if they can trust their own co-workers or even each other for that matter and Luk's testimony to what he saw poses a threat to the criminals involved in the conspiracy. Now for the next 72 hours, Penny and Donny will have to learn to tolerate each other while trying to protect Luk from the assassins who want him silenced. If they can do that, the bad guys don't stand a chance and they just might all survive this case.. if they don't end up killing each other first!