The Undercover 1992 on netflix

Chik Chi Ming is promoted to senior inspector after solving a major case while in prison. However, he learns that his old friend, Sai, who was released and has returned to a life of crime. To help him reform, Chik arranges for Sai to go undercover in a notorious gang. Sai earns the trust of the gang leader’s mistress and becomes her driver, but discovers her affair.