Lawrence plays Agent Malcolm, a svelte, young FBI man who prefers to live as an obese, elderly woman. As a result of his unusual personal life, Malcolm lacks close human relations. The nearest thing he has to a friend is his partner John, played by Paul Giamatti. John, however, would rather spend time with his family than with the bizarre Malcolm, thus setting up a painful contrast: Malcolm, though inherently maternal, has no family, for what family would accept his alternative lifestyle?
Luckily, the perfect assignment falls into Malcolm's differently gendered lap: He is to disguise himself as "Big Momma," the grandmother of a bank-robbery suspect who is on the lam.
When the suspect, the beautiful Sherry (played by Nia Long, here taking a break from acting in order to trade on her beauty), arrives at the titular Big Momma's house with her young son Trent in tow, she settles in, giving Malcolm for the first time what he has always dreamed of: a family, with himself as the matriarch.
One of the most realistic and convincing moments of the film occurs when Sherry first arrives at Big Momma's house (the real Big Momma is out of town, thus allowing Malcolm to take her place): she sees Malcolm in rubber mask, padded dress and ridiculous wig, and is convinced that this 35-year-old man is actually her 90-year-old grandmother. Who wouldn't be? Being a 90-year-old woman isn't in the bone, it's in the heart, and Malcolm is all heart.
In fact, he takes immediately to the role of mother, nurturing Sherry and Trent, while simultaneously ogling when Sherry walks around the house in her extremely realistic Frederick's of Hollywood lingerie.
He also keeps trying to get Sherry to confess to the bank robbery, worming his way into her confidence by pretending to be the woman who raised her. Sherry grows closer to Malcolm/Big Momma, even cuddling up in bed with her, but she won't let on about the robbery.
However, in the bed with Sherry, something stirs for Malcolm; an aspect of his long neglected sexuality is awakened by the close contact with the comely Sherry. So much is his female side subdued by this supremely sexualized encounter that Sherry, her back pressed up against the disguised Malcolm, must ask if there is perhaps a flashlight hidden between Big Momma's thighs.
At this point Malcolm's gender dystonic disorder must confront the growth of his amorous feelings towards Sherry. Does he wish to be her grandmother or her lover--or both? And what about the whole FBI thing? Shouldn't he be trying to arrest Sherry?
As luck would have it, the cops-and-robbers story takes up only a few minutes of film time, so mostly Malcolm seems to be simply living out his dream life as a giant old woman, rather than engaging in the morally questionable activity of entrapping a criminal. Malcolm/Big Momma takes Trent fishing, goes shopping with Sherry, and helps the other elderly women in the North Carolina town where he/she lives with baking and midwifery needs. I'm pretty sure at some point there's some gunplay and the actual plot is dealt with, but for the most part that stuff is wisely set aside in favor of a series of disconnected, one-joke scenes.
Which is not to say that Big Momma's House is actually funny. Just as the murder of Brandon Teena in Boys Don't Cry was presented as a tragedy, not a comedy, there is a tragic lack of jokes in Big Momma's House, thus educating the audience with the knowlege that, whatever else may be said about age dystonic gender confusion, it is no laughing matter.