Narcotics Bureau detective Wai is sent to jail for planting evidence on a drug raid. His sergeant, Sam, is given disciplinary action as well.
Seven years later, Sam – now a member of the Serious Crime Unit – encounters Wai while pursuing a suspect. To redeem himself for his part in his former partner’s downfall, Sam destroys all the evidence that incriminate Wai. When one of Sam’s superiors suspect wrongdoing, Sam lies and says that Wai is working undercover for him.
Caught between a rock and a hard place, Wai has no choice but to go undercover, but he is soon discovered by the head of the cartel. Sam’s daughter is killed in a traffic accident, and Wai is attacked in his home.
Without any hard evidence, Sam and Wai are forced to go back to their old ways…