In Infernal Affairs III, Tony Leung reprises his role as undercover cop Lau Kin Ming, who is now a high-ranking member of the Triad. The film follows two timelines: the first takes place before the events of the first film, with Lau struggling to balance his duties as an undercover cop with his loyalty to the Triad, while the second takes place after the events of the second film, with Lau haunted by his actions and seeking redemption.
The film also introduces new characters, including Chen Wing Yan's former love interest, Dr. Lee Sum Yee (played by Kelly Chen), who is now a psychiatrist treating Lau. Anthony Wong also returns as triad boss Wong Ka Yan, who is now facing challenges from both the police and other triad members.
Infernal Affairs III explores themes of loyalty, identity, and redemption, and features complex character arcs and a gripping plot with plenty of twists and turns. While not as widely acclaimed as the first two films, it is still considered a worthy addition to the series and a compelling crime thriller in its own right.