The Witness starts off with a gun shot, leaving one victim dead. Sergeant Ko Lik Kei is tasked to protect the case’s most important witness Lee Ho Fan. Kei and Fan struggle to get along, but in times of need, they find that they are always there for each other. Meanwhile, Ho Fan’s company gets involved in another criminal case. The company’s executive Yip Cheung Lok and his son Yip Ming Kwan become key proponents. Kei, Fan, and Fan’s younger sister Lee Chung Ying, who is a lawyer, investigate the company together, becoming a dynamic trio. Kei’s older sister Ko Wing Yam also gets pulled in the ropes.