Cheung Sai-Lun (Vincent Wong): a struggling actor from Ohio with Hong Kong heritage comes back to Hong Kong to audition for a role in a film. However, he is wrapped up in a murder case involving the murder of triad boss: Old Chiang and his 2 sons, he is asked by Rachel Koo (Crystal Fung): a senior inspector of OCTB to become a spy in a triad for the police alongside the pre-existing spy: Ko Ban (Owen Cheung). He is "cast" as the Chiangs' long lost third son with the fact that the real son had been killed in a gang war beforehand. Cheung soon gets acquainted with the three Chiang sisters: Chiang Chin-Ha (Winki Lai), Chiang Chin-Yu (Kelly Fu) and Chiang Chin-Wai (Serene Lim) as well as their uncle: Lui Ka-Cheong (Lee Shing-Cheong) and begins living with them. He also starts intimate friendships with celebrity actress Angel Chong (Samantha Ko) whose manager: Koo Wing-Cheung (Shek Sau) is the father of Racheal, Cherry So (Kathy Yuen), a high school teacher and daughter of one of the triad leaders: So Wai-Sun (nicknamed Old Fox) (David Chiang) and Yiu Ching-Shui (better known as Shui Je) (Angel Chiang) who is sent by her boss: Chung Chi-Sum (Tsui Wing) to seduce and spy on Chiang only to fall in love with him instead. Throughout the series, both Cheung Sai-Lun and Ko Ban begin to discover more truths about the murder and begin to suspect that the real culprit may just be in the police force. Meanwhile, both Chung Chi-Sam and So Wai-Sun begin plotting to oust Chiang and become triad boss themselves.………
