Infernal Affairs III

Infernal Affairs III is a 2003 Hong Kong crime thriller film directed by Andrew Lau and Alan Mak. It is the third and final installment in the Infernal Affairs film series, and is both a sequel and a semiprequel to the original film, as it intercuts events before and after the events in the first film. Andy Lau, Tony Leung, Kelly Chen, Anthony Wong, Eric Tsang, and Chapman To reprise their roles again, joined by new cast members Leon Lai and Chen Daoming.

Undercover cop Chan Wingyan seeks to uncover the link between Hong Kong triad boss Hon Sam and the mysterious Chinese gang leader Shen Cheng. Since Hons ascension to the seat of triad boss was due to the death of his predecessor, Ngai Winghau, Hon is suspicious of all his men for fear they might usurp his position. He tests Chans loyalty by ordering him to smash an ashtray on Shens brother during a negotiation, resulting in Chans arrest by Inspector Yeung Kamwing. Yeung tells Chan that though Chan does not recognise him, he recognises Chan and he warns the latter to be careful. Chan is released after both Hon and Shen fetch him at police headquarters.Concurrently, Chan is prosecuted for violent behaviour. His superior, Superintendent Wong Chishing, persuades the court to allow Chan to seek therapy, leading him to meet Dr. Lee Sumyee. Hon asks Chan to deliver arms to Shen, but he and other triad members do not show up. When Chan delivers the cargo, Shens men discover that the boxes are empty and open fire on Chan Shen and Chan shoot each other in the limbs during the crossfire. Shen finds out that Chan is an undercover cop when Yeung unexpectedly arrives on the scene. Yeung tells Chan that Shen is also an undercover cop, working for Chinese authorities. Yeung also tells Chan that he gained top honours when he was in the police academy due to Chans expulsion. The three shake hands and wait for the chaos to subside before leaving. ........

Source: Wikipedia