2017年，李文彬（梁家辉 饰）突然失踪，与此同时，蔡元祺在英国惨遭暗杀。为此，刘杰辉（郭富城 饰）向简奥伟（周润发 饰）寻求协助，开启了一份1994年的神秘档案。
1994年，香港回归前夕，政治部即将解散。一宗富商绑架案，让热血正义的李文彬（刘俊谦 饰）与冷血野心家蔡元祺（吴彦祖 饰），在警界掀起一场暗潮涌动的权力较量。英方、警队、富商、黑道四方势力卷入其中互相博弈，香港迎来了新一轮权力洗牌，并为2017年的寒战风波埋下了伏笔。

Tom is forty. He walks out one day, rather abruptly, on his wife and baby boy and his seemingly happy life. He finds himself living on the streets of London. One night alone in a park he is mistaken for a gay man and is set upon by a gang of violent thugs.
In A&E the next day Tom meets Aidan, the happiest, fast-talking individual you are ever likely to meet, the complete opposite of Tom. Too polite, or too weak to ask him to leave him alone Tom tries to get away from this child-like man but with little joy, Aidan sticks to Tom like glue.
Tom reluctantly becomes involved in Aidan's life and he quickly realizes that Aidan has problems too. Aidan's 'girlfriend' Linda verbally and physically abuses him on a regular basis. Will Tom overcome his own problems in order to help his new 'friend'? Will Tom ever make it back home - and why exactly did Tom leave home in the first place?