When a cop expresses his grievance and anger onto the world, he becomes the most dangerous man on the force. Taken from the top ten famous unsolved cases in Hong Kong, Gun Metal Grey depicts a cop's road to destruction as well as the very truth of human nature. Fifteen years ago, upright police officer Shek Tung-sing was falsely accused of murder and sentenced to prison. Now that he has been discharged, he decides to get to the bottom of the case. Unfortunately, he is unable to bring the culprit to justice because of lack of evidence.As a result, his suppressed anger continues to churn within him and whenever the law fails to carry out justice, he takes matters into his own hand.The sudden change in his character and behavior places his friendship with his teammate Mai On-ting to the test. Later, when he is trying to prevent Ting from looking further into the case, he even hurts Ting's girlfriend, Hui Man-him. The competition begins to intensify between the two good friends when Ting is determined to deal with Sing according to the law.