This is a story about Hong Kong Customs. The trio comprising Intellectual Property Investigation Bureau Senior Superintendent Hoi Fung (Benjamin Yuen), Trade Controls Bureau Chief Trade Controls Officer Kwan Pui-yan (Mandy Wong) and Customs Drug Investigation Bureau Superintendent Wan Tsz-ki (Venus Wong) lead a contingent of elite customs officers and fresh-faced rookies, including Probationary Inspector Chan Ka-hing (Carlos Chan), to carry out diverse operations involving eggs, nuclear bombs, baby formula, heroin and much more. They are always ready to go all out not only on the ground, but also in the sky and at sea, showcasing a reinvented and ingenious Customs and Excise Department. This group of individuals adhere to the motto "Serve the Community with Commitment and Excellence". Although they have their own issues, they can still overcome hurdles. They contribute to social stability and prosperity in Hong Kong, and Hongkongers are proud of The Line Watchers.