保龄球（曾志伟 饰）和花枝康（林俊贤 饰）、通天明（许冠英 饰）等一干好友相识多年，彼此之间感情十分深厚，某日一行五人参加了一个前往泰国的旅行团，哪知道遭遇了意外竟然被迫成为了试药对象，药物令除了保龄球之外的四人统统变成了小孩，四人只得暂时躲在保龄球的家中。 　　通天明的老婆鱼雷艇（林建明 饰）觉得丈夫的不知所踪十分可疑，再加上四人的银行账户都攥在保龄球的手中，不禁怀疑保龄球谋财害命，随后闹出了不少的笑话。之后，鱼雷艇和花枝康暗恋的王老师（陈雅伦 饰）得知了事件的真相，与此同时，五人之前结识的神医终于调配出了解药，这场返老还童的风波眼看着就要平息。

Five orphans, five friends. They grow up together, still good friends, and travel to Thailand, where a witch doctor has invented a brew which returns one to childhood permanently. Robberts attack and force the five to test the brew. Rescue comes after four of them have shrunk into child sizes, leaving Bo the only adult. The doctor forgets how to reverse the process and the quintet return to Hong Kong. Teacher Wong takes the four children into a primary school.