A drug cartel boss is arrested in a raid and coerced into betraying his former accomplices as part of an undercover operation.
A brilliant detective is forced into early retirement after losing eyesight. Making ends meet by solving cold cases for reward money, he teams up with a rookie lady inspector to solve a case from her personal past.
When Hong Kong is rocked by multiple gruesome murders, the police forms a task force to investigate. Jun, once a brilliant detective who suffered a mental breakdown, begins his own investigation. Eventually, the police learn that the murder victims are all suspects of cold cases being rubbed out by a figure known as "The Sleuth". Now, Jun and a detective from the task force are on a race against time to beat the brutal killer at its own game.
