In In the Line of Duty IV, Inspector Rachel Yeung (played by Cynthia Khan) teams up with Seattle cop Donnie Yan (played by Donnie Yen) to track down a group of drug smugglers after a dock worker becomes an unwitting witness to a crime. The investigation stretches from Hong Kong to Seattle, uncovering corruption, betrayal, and a dangerous international drug syndicate.

The film combines fast-paced martial arts sequences with explosive action scenes. Donnie Yen and Cynthia Khan deliver intense physical performances, and the film is known for its realistic stunts and hard-hitting choreography, a trademark of director Yuen Woo-ping.