FBI探员马刚(马丁•劳伦斯 Martin Lawrence 饰)最近和妻小闹得很不愉快,家人总是责备他太忙于工作而疏忽了照顾家庭。马刚这边正闹得情绪低落,却又有大案件找上了他。原来某日政府机关的电脑突然全部死机,过后发现很多绝密文档都被人浏览过了。FBI怀疑电脑工程师汤姆(马克•摩斯 Mark Moses 饰)有重大嫌疑,却又一直都找不到证据,于是只好派马刚再次出山。军命难违,本来不愿意再增加工作的马刚只有再次化身成肥胖的老妈子,以保姆身份混进了汤姆的家。在这里,他却意外发现了一个喧哗的世界,他和家里的三个孩子玩得不亦乐乎,亦知道了重拾天伦之乐的方法……Undercover Agent Malcolm Turner is back and this time he's out to expose the suspected designer of a deadly computer "worm" that would allow outside forces access to sophistical and critical government intelligence files. But unfortunately, the only way the crafty agent can get next to the worm's creator, Tom Fuller, is to access the programmer's Orange County home as the new "nanny" to Fuller and his wife Leah's children: toddler Andrew and his two older sisters, Carrie and Molly. This means that Malcolm must once again rely on his sure-fire alter ego, the take-no-prisoners Hattie Mae Pierce, a.k.a. "Big Momma," to bring down the bad guys and prove that a woman's work is never done! But once undercover, the job proves another tough juggling act for Big Momma as "she" must manage the hectic lives of the three Fuller kids, keep up with their myriad of daily activities, and handle the many household chores, all while secretly trying to dig up information on Tom's computer virus. Of course, Malcolm's hardest and most comical task is staying in Big Momma character throughout, which requires the trickiest of physical and vocal transformations, not to mention the ability to think really fast on his pumps. Hilarious situations ensue for "Big Momma" while trying to trap her prey, including, infiltrating a ritzy women's day spa, leading a bawdy teen cheerleading competition, and surviving a crazy jet ski chase, all while trying to keep "her" wig straight and skirts smooth. Along the way, "Big Momma" also helps bring the fractured Fuller clan back together and, in turn, Malcolm develops a newfound appreciation for his own family, which includes stepson Trent and expectant pregnant wife Sherrie. Malcolm also learns he can strike a successful balance between his work and home responsibilities, without shortchanging either side in the process.