Mainland cop travels to Hong Kong to team a Hong Kong police officer in order to investigate a case involving a brutal killer.

The Hong Kong detectives, Chau and Beethoven, are back once more. This time, their mission is to protect a young girl from a gang boss' henchmen, who are trying to eliminate her so she couldn't testify in court against their boss. Meanwhile, the plot thickens when a police medical report revealed that Chau apparently has cancer; therefore, Beethoven will see to it that his partner will have a well-deserved vacation in his remaining months.