Australian Chinese gangster "solver" K (Wong Ho-sun) wants to stay away from the underworld. His beloved Chloe (Cheung Nai-man) dies accidentally and is framed and hunted down. K goes to Hong Kong to become a secondary school physical education teacher OSir, starting a second life. The new head teacher of Class 5E, Jovy (Natalie Tong), is actually the daughter of the gang leader. Her father's retirement from the underworld has caused a huge wave in both the black and white worlds, and has also affected the campus. O then protects the people around him as a solver. Inspector ManSir (Cheung Wing-kang) is determined to eradicate the underworld forces. A truth makes his moral values collapse and he embarks on the road to becoming a demon! Jovy's father was assassinated. In order to balance the chaos, he took over the position of leader, and O assisted in protection. The world of good and evil has been repositioned. Who can set things right?