When he was younger, Ko Yat Tai infiltrated the triads as an undercover agent. The triad leader's daughter, Lou Man Na, fell in love with him at first sight. Although Ko Yat Tai already had his heart set on Lee Ying Lan, he pretended to be with Lou Man Na to solve the case During a fight between triads, Ko Yat Tai eliminated Lou Man Na's triad rivals, but Lou Man Na's fate remained unclear. Ko Yat Tai's good friend, Nip Yu Hong, knows about his affair with Lou Man Na and their infant son Ko Ka Long. Ko Yat Tai brings Ko Ka Long home, and Lee Ying Lan also gives birth to a son Ko Ka Chu. The half-brothers accumulate a number of grudges growing up. Both also coincidentally develops a father-like mentor relationship with Nip Yu Hong. During a promotion ceremony, Ko Yat Tai, having achieved the rank of Superintendent, is happily preparing to receive this honour but unknowingly steps into a revenge plot that is 30 years in the making.

An unknown terrorist group is behind a series of crimes in Hong Kong, China, putting the safety of the city and its citizens on the line. A special task force, A-Team, is formed under Lok Ka-sing, the captain of the Flying Tiger, with selected elite members from the police force: Head of operations Eddie Wong Kwok-tung, head of investigations Cheung Man-lung, heads of intelligence Ting Ho-yin and Mo Yat-man, and head of forensics Yiu Ka-lam.

The terrorist organization “Argunbia" infiltrated Hong Kong and set off a battle for chemical weapons in Hong Kong. Fan Siu-fung (Moses Chan), the commander of the Special Duties Unit (SDU), was ordered to fight against the terrorists. The SDU are trapped in the ruthless killing of “Wakanbo”, leading to the Special Service Superintendent Chin Bok-man (Joe Ma) to assemble the police elites Cheung Wai-wah (Eddie Cheung), Cheung Ka-hin (Bosco Wong) and Ko Chi-lok (Ron Ng), to form a secret counter-terrorism team: S Team. In the face of many crises, with the assistance of Interpol Hui Chun-fei (Michael Miu), the generations of their grievances and feuds are put aside and they join forces to prevent the spread of the chemical weapon virus in Hong Kong.