The series takes place a year after the 2nd installment.

Chan Siu Sang is single and all alone, his old love Chu So Ngo married the doctor that treated her son and left the police force. Then he meets Wai Ying Zi, a rookie police officer that is under Chan Sam Yuen Police Traffic Department unit. The two get off on the wrong foot due to misunderstandings. Wai Ying Zi who is eager to show off her skills seems to get into a lot of trouble. She is deemed reckless and rash. Later on, Ying Zi joins Siu Sang's CID unit which leads to the two bickering non-stop. After Siu Sang saves Ying Zi life on one of their investigations she learns to respect him and the two become friends. Chan Sam Yuen is promoted to the head of the Police Traffic Department. She and Ching Fung are also now parents to a pair of girl/boy twins, but their marriage is not as happy as it seems as the two continue to have quarrels in their marriage. With marital problems at home, Sam Yuen becomes a cold, unreasonable and mean boss at work. When a case similar to the psychotic serial rapist in the last series arises Sam Yuen starts feeling threaten and insecure even though the culprit Bao Gwok Ping is still locked up in jail. It seems Bao Gwok Ping has been communicating with someone on the outside who agreed to take on his split personality persona and continuing his crime spree. Due to Sam Yuen's carelessness her son is kidnapped and killed by the new culprit. This causes Sam Yuen and Ching Fung marriage to be in further jeopardy when he blames her for the death of their son.