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...
