Cheung Dai Yong, a brilliant and astute police officer, has a track record of solving crucial cases. His trusted colleague, Lee Chung Yee, stands by his side, and together they tackle challenging and complex cases. Amidst their work, Dai Yong encounters Ko Jit, a tenacious journalist. Despite their initial conflicts, a deepening connection blossoms between them. However, Ko Jit already has a romantic partner.

Detectives Cheung Tai Yung and Lee Chung Yee are back! This time, they are faced with even tougher cases: “Fatal Attraction”, “Destroying the Corpse”, “Death Warning, “ and “The Son’s Cross”. Meanwhile, Yung and Jessie are at a crossroads. Their love is tested as Carmen, a new member of Yung’s team, comes into the scene. Carmen is crazy about Yung. Then, Jessie finds out Carmen was once involved in a murder case.

An even more thrilling succession to the second series, the "Detective Investigation Files III" continues to depict mysterious murder cases solved by Yung (Dai Tai Yu) and Yi (Leung Wing Chung), while at the same time, goes on telling the love story of the two detectives.

Detective Investigation Files IV (Chinese: 刑事偵緝檔案IV) is the 4th and final installment of the Detective Investigation Files TV franchised by TVB in Hong Kong. It is a complete reboot of the series, with new cast. The series focuses on the lives of two detectives, Tsui Fei (Louis Koo) and Kong Chi Shan (Sunny Chan), and the murder cases they deal with.