Young police officers Owen Cheung, Sisley Choi, Gloria Tang and Brian Chu apply to join the Airport Security Unit (ASU) for different reasons. Under the strict guidance of instructor Eddie Kwan, they meet the standards for graduation and join ASU. Owen frequently butt heads with ASU's Chief Inspector Hugo Wong. Sergeant Matt Yeung, knowing about their animosity, often acts as the mediator. Owen encounters air stewardess Roxanne Tong. The pair fall in love easily, but find it hard to get along. Meanwhile, Owen gradually develops feelings for his colleague Sisley. As they carry out their duties, they witness joy, sorrow, farewell and reunion. They linger on the edge of life and death, and begin to understand the sense of mission of the ASU! A great danger slowly approaches and throws the team into unprecedented difficulties...