Laughing is convicted of killing crime boss Michael So and receives a life sentence. While incarcerated, he encounters Fok Tin Yam, a seemingly gentle fellow prisoner and former university professor specializing in criminal psychology, who is serving time for drug-related charges. Fok, with his sharp intellect and manipulative skills, demonstrates his prowess by easily overpowering the aggressive prison bully. He also figures out that Laughing is actually on a covert mission to uncover corrupt officers within the police department. The investigation points to Chit, an ex-sergeant from the Narcotics Bureau who is battling a mental illness. To extract vital information from the unstable Chit, Laughing needs Fok’s assistance, but Fok has his own mysterious motives…