 In the prequel of Infernal Affairs. Chan Wing Yan has been expelled from police academy in cause of his relatives to the triad. Now SP Wong give him a chance to undercover the triad family controlled by his half brother Hau. Besides of Ming. He has been ordered to killed Hau father and infiltrated the police department. The story get complicated when Wong s related to Hau father s dead. The avenge is begin when Mary. Sam s wife is the hit order. Now everything is complicated and related