Ye ye ban fei jiao (1992)

Two mismatched Hong Kong police detectives, Yau and Yue, were assigned to work undercover as a live-in couple to track down a serial killer accused of murdering adulterers. In the meantime, Yau (Anthony Chan) is coping with his girlfriend having left him, and Yue (Sandra Ng) is losing her boyfriend to a college girlfriend of his.