香港警察朱福龙（甄子丹 饰）因一次工作失误惨被降级，万般无奈之际又遭未婚妻可儿（周励淇 饰）悔婚，事业爱情的双重打击令他一蹶不振，暴饮暴食六个月后体重猛增到220磅！上司黄警官不忍其继续堕落，委派朱福龙前往日本押送一名犯人，并承诺只要完成任务就可以为他恢复职位。可惜来到日本，倒霉的事件便接连在他身上发生，不仅财物尽失，连犯人也在押送途中逃脱了。幸好得到前香港警察潇洒哥（王晶 饰）的帮助才得以暂时解困，为了完成任务找回犯人，朱福龙与潇洒哥共同行动，不料在查找真相的过程中发现了日本黑帮的惊天阴谋......Fallon Chu (Donnie Yen) is a Hong Kong Police Special Task Force agent. An agile fighter with a high crime detection rate, he's considered a role-model in the eye of both the police force as well as the public. He gets assigned to a case of escorting a well known criminal to Japan using his experiences and fighting capabilities to keep it safe during the job. Unfortunately, things are not going well in his private life as he has to deal with problems with his fiancee while planning their wedding. After a series of mishaps between them, she suddenly dumps him. Heartbroken he indulges himself in food to relieve his state of shock and loneliness, and in three month' time his endless intemperance turns him into a 300-pound being.