Off-duty police officers Zhang Jing Shan, Ma Rong Yi, Zhang Zhen He, Xin Kai Qing and Jiang Hao Zheng happen to arrive at the airport to participate in the rescue operation and arrest of a suspect. One year later, Jing Shan, Kai Qing, Shi Hua and Brian Chu who belonged to different police departments report to ASU at the same time.

Braving four days and three nights of selection process followed by twelve weeks of rigorous training under the strict guidance of Man Sir, the four forge a strong friendship as they are integrated with the old members of ASU. Encountering different situations on duty, they see the joys and sorrow of life that make them reflect on their own lives. With every step forward, they establish a sense of mission as the airport security police.