Tough cop Ken Li (Cheng) is at the top of his game, but he's going to need some help to solve a grisly crime that the offender, a respected police officer, doesn't even remember committing. He seeks out Jack Lai (Lai), a former psychologist now in disgrace and behind bars, who has an uncanny ability to bend people to his will. Making an uneasy truce, this unconventional pair set out on a mission to find the man responsible, a master of hypnotism who will stop at nothing to get his revenge!

Une bande de jeunes ados de la haute bourgeoisie aimant les sensations fortes, braquent spectaculairement une des plus prestigieuses banques de Hong Kong, et restent sur les lieux du méfait, juste pour le plaisir de pouvoir tirer sur les forces de l'ordre au fusil mitrailleur. Suite à ce jeu de massacre, l'inspecteur Wing promet publiquement de retrouver les assassins en moins de trois heures.

Laughing is judged guilty of murdering gang lord Michael So and is sentenced to life imprisonment. Locked up behind bars, he meets the seemingly benign inmate Fok Tin Yam, a university professor in criminal psychology who is jailed for drug possession. Highly intelligent and manipulative, Fok reveals the power of his mind games by effortlessly subduing the violent prison bully. He also deduces that Laughing is sent to the prison on an undercover mission to find out the rogue cops within the police force. The clues lead him to inmate Chit, a former sergeant from the Narcotics Bureau who has been suffering from a mental disorder. To unlock the critical information from the deranged man, Laughing must get the help of Fok, but this enigmatic figure has an agenda of his own...

Best Friend of the Cops is a Hong Kong Action movie starring Jacky Cheung and Alex Man