澳洲华人黑帮「解决师」K（王浩信饰），欲远离黑道，挚爱Chloe（张曦雯饰）意外身亡且被嫁祸追杀。K到香港成为中学体育老师O Sir，展开第二人生。5E班新任班主任Jovy（唐诗咏饰），实为黑帮龙头千金，父亲金盆洗手令黑白两道掀起巨浪，且殃及校园，O再以解决师身份保护身边的人。督察Man Sir（张颖康饰）矢志铲除黑势力，一个真相令其道德观崩溃，踏上成魔之路！Jovy父被暗杀，为制衡乱局接管龙头之位，O协助保护。正邪世界重新定位，谁能拨乱反正？
Jim Phelps is the new leader of the Impossible Missions Force, with no explanation, or even reference made, about the departure of predecessor Dan Briggs. First up for Phelps is a mission involving the drug trade. Cresnik and Walters have cornered the heroin market and sell it at a handsome profit to gangs around the world. Phelps devises an operation which will drive a wedge between the heroin dealers and their customers.
