Sergeant Ko Fai completed undercover operations many years ago, and there is always a cloud hanging over him. Ko Fai is reassigned to a police precinct that is Crime Squad Station Sergeant Fong Chi-kiu’s territory. While investigating cases, the duo gradually familiarize with each other and develop tacit understanding. Chi-kiu is so dedicated to her job that she has no time for taking care of her family. Her son hangs out with the wrong crowd and gets into trouble. Hotpot restaurant lady boss Lo Sin-man was naive when she was young as she slept with gang boss Law Wai and gave birth to a girl called Lo Siu-po. Fortunately, Ko Fai has always taken care of Sin-man and her daughter. Siu-po becomes a KOL and wants to step into the spotlight by engaging in reckless maneuvers. She gradually descends into the abyss as she reveals how drug dealers go about their business. To protect their loved ones, Ko Fai and Chi-kiu become guardians of the night and are determined to eradicate the villains!
沙展高輝完成臥底任務多年，一直走不出陰霾。高輝被調往重案組警署警長方梓翹所屬警區，二人於查案期間逐漸磨合，建立默契。梓翹工作拼搏，難以兼顧家庭，兒子更與小混混結交而陷入危機。火鍋店老闆娘盧倩雯當年少不更事，為社團頭目羅威誕下女兒盧小寶，兩母女幸得高輝關顧。成為KOL的小寶為博出位不惜犯險，揭露毒品交易情況，漸漸步入無底深淵……為了保護摯親，高輝、梓翹當上黯夜的守護者，誓要將黑勢力連根拔起！
警察担任黯夜守护者，守护着这个城市，亦透过不同案件，揭示案中人背后的故事，探索社会阴暗一面。除了案中受害者，陈展鹏、陈炜、陈敏之等所演的角色也是有故事的人，以往的遭遇影响着生活，每个选择亦影响将来的人生。面对人生种种纠结与关系上的爱恨交缠，该如何作出正确抉择，寻找人生出路？
