Infernal Affairs (2002)

Chan Wing-yan, a police officer, goes undercover into a Triad (underground society); only his direct superior, Superintendent Wong, is aware of his mission and true identity. Around the same time, Lau Kin-ming, a triad member, infiltrates the Hong Kong Police Force on the orders of a powerful gang boss, Hon Sam. Each Mole (espionage) has been planted by the rival organisation to gain an advantage over the other side. Over the course of ten years, Chan experiences great stress from his undercover work while Lau quickly rises through the ranks in the police department.