Martin Lawrence returns as FBI agent Malcolm Turner, who again goes deep undercover as "Big Momma." The FBI has learned that a computer software developer named Tom Fuller has created a computer virus that allows access to classified US government files. Fuller is planning to sell the virus to terrorist organizations all over the world. Upon hearing this, the FBI's best chance is Fuller's wife. So Malcolm goes undercover as Big Momma in order to pose as Mrs. Fuller's nanny and as always, Big Momma turns the house upside down.
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 government intelligence files. But unfortunately, the only way the agent can get next to the worm's creator is to access the programmer's Orange County. At home is Malcolm a 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 bring the bad guys down and prove that a woman's work is never done! All this while Malcolm 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. This means physical and vocal transformations. 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 short changing either side in the process.