FBI探员马刚（马丁•劳伦斯 Martin Lawrence 饰）聪明能干，屡破奇案，而他的破案法宝就是他那高超的伪装绝技，无论什么人，他都自信能扮得惟妙惟肖。新的案子摆在了马刚面前，乔治亚州一个银行抢劫犯身怀巨款逃亡了，马刚奉命前往缉拿，唯一的线索就是抢劫犯的女友雪梨（尼娅•朗 Nia Long 饰）。为了接近雪梨，马刚施展他的变身特技，化身成了雪梨久未碰头的姑母，一个超过两百磅的“老妈子”。马刚的特技堪称一绝，经过化身易容，成功接近了雪梨。然而相处久了，马刚却发现自己爱上了可能有份参与收藏赃款的雪梨，更要命的是，真正的姑母登场了。FBI agent Malcolm Turner is known best for being a brilliant, master of disguise. Malcolm's latest assignment sends him to small-town Georgia, where he's assigned to trap a brutal bank robber (and a recent prison escapee) who they suspect will be coming down to visit his ex-girlfriend Sherry and her son. Malcolm sets up a stakeout across from the home of a larger-than-life southern matriarch known as Big Momma, who's about to be visited by Sherry. It's a simple plan, but there's one big problem: Unbeknownst to Sherry, Big Momma has unexpectedly left town. So Malcolm, decides to impersonate the cantankerous Southern granny. Using a few tricks of disguise, he completely transforms himself into Big Momma, even taking on the corpulent septuagenarian's everyday routine-from cooking soul food to delivering babies to "testifying" at the local church. In the mean time, Malcolm starts falling for Sherry, who may or may not be hiding some stolen cash. Now, Malcolm/Big Momma must somehow find a way to nab his criminal and the girl.