Infernal Affairs 3

Infernal Affairs III picks up where the first film left off. Ming is cleared of any charges involving Yan's death and is eventually assigned to the Internal Affairs division. He discovers that another cop, Yeung, quickly rising through the ranks of the police department, has a mysterious link to Shen, who was apparently Sam's connection to the mainland.
Ming strongly suspects that Yeung is another one of Sam's moles and is determined to expose him while keeping his own connection to Sam a secret. It's a tricky proposition because Yeung also seems to suspect Ming and appears to have the same goal in mind.
With the help of Dr. Lee, Yan's psychiatrist, Ming looks deeper into Yan's final days, and flashbacks explore the undercover cop's dealings with both Yeung and Shen. Eventually, Ming finds an incriminating tape of Sam conversing with his mole and has a climactic confrontation with Yeung.