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... (source: imdb.com)