海关高级督察刘庆宗及其徒弟张志敏是走私集团的罪恶w星，令走私集团闻风丧胆，庆宗多年来一直寄钱资助泰国一个叫梅宝的少女，但梅宝的父亲却挑拨她暗杀庆宗……
陆地兵营的Ken和罗邦改为到蛙人海军处报到，以及他们在训练时遇上了香港移民过来的黑社会石黑龙（黄恺杰饰）以及人称天下第二的蛙人教官（张智扬饰）所发生的故事。
梁志强宣布将开拍《新兵正传4》，《新4》计划2017年开拍，前面三集的大部分演员都将参与。 梁志强受访时说：“《新兵正传3》反应热烈之后，很多影迷都渴望我们可以尽快开拍《新兵正传4》。“他说第四集将以战备军人为主题，至于属于哪个兵种，他还在与国防部研究，不过大家可以放心的是，不用担心故事会重复，或是与过去三集有相似的地方。
A group of tired commuters have caught the last train home, ready to hop into their beds for the night. However, their train soon veers off-course and crashes into an abandoned tunnel. The three surviving commuters; withdrawn mother Yi Ling, her son Lucas, and student Janice, are forced to band together and survive the night, as well as avoid the mysterious monster hunting them. Yi Ling still suffers from flashbacks to the car accident that killed her husband and nearly took Lucas` life too. When Lucas is snatched away by the monster, her maternal instincts are tested, and Janice`s ideas of family are challenged as she remembers her difficult relationship with her estranged father. Back at the subway control headquarters, the engineers do their best to make sense of the situation and rescue the survivors.