In the Line of Duty 4 (1989)
"Wong Ka Si Sei IV: Sik Gik Sing Yan" (original title)

Great addition to the series.

The film opens in Seattle where, after meeting Cynthia in the obligatory opening kung fu scene, an innocent dockworker, Luk, accidentally runs across some corrupt CIA agents making a double-cross for a secret film negative. In a Hitchcockian twist, the negative is thought to be in the Luk's hands, when it actually has rolled into the water. Of course, no one believes the unlucky Luk doesn't have the film negative- not the CIA and not Donnie Yen, Michael Wong, or Cynthia, the cops on the case, who export Luk back to China. It is early on that Donnie is established as the hot tempered one, Cynthia is just Cynthia, and after playing a good guy in Royal Warriors (In the line of Duty 1) Michael Wong clearly is playing the good looking, but underhanded bad cop in league with the corrupt CIA. Despite trying to wine and dine Cynthia, she suspects Michael is a turncoat, and it takes awhile, and a lot of action for her to convince Donnie that Michael, his friend, is bad. All the while, they have to deal with protecting poor Luk from the renegade CIA agents at every corner.

The action (thanks to the great Yuen Woo Ping) is typical of the series, inventive and brutal, but what sets this one apart is the sheer number of action scenes. There are three kung fu fights and a shoot-out in the first fifteen minutes. In total (Yes, I counted) there are fourteen kung fu battles, three shoot-outs, a brief ice locker torture scene, a car bomb, two scenes involving hitmen on motorcycles, and two interrogation room beatings. Cynthia's highlights are a great fight on a moving ambulance in witch she is shoved though the window, her head dangling above the pavement, fighting on top of it, hanging off the front grill, and another fight with a fugly gwailo woman in a warehouse that involves some precarious scaffolding and elevator shaft fu. Donnie, however, has a the majority of the good fights, like a motorcycle chase and joust, as well as his fantastic final fight with a beefcake Ike Turner lookalike. If you are looking for action every three minutes, look no further.

The In the line of Duty series is mainly known for showcasing the talents of female action stars, Michelle Khan (Yeoh) and Cynthia Khan, however this entry is mainly Donnie Yen's chance to shine and one of the first films to gain him any notoriety as a lead. Before this films success, he was essentially going to give up hope for a movie career. Donnie actually saves Cynthia in most of her scenes. When she is drugged and fighting a knife wielding assassin, its Donnie to the rescue! When they are attacked by a motorcycle psycho hitman, its Donnie who takes him on. When Cynthia is fighting Michael Wong in the finale, it's only with Donnie's help that they finish him. So, its rather clear that Woo Ping favored Donnie, and, no disrespect to Cynthia, but kung fu fans are all the better for it.