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Martin Lawrence goes undercover in the least convincing padded housedress this side of Mrs. Doubtfire’s, but the actor’s impeccable timing and shrewd eyes, not to mention the character’s grandma-patented admixture of Southern sugar and old-broad vinegar, make the deception work, even if the plot is as half-baked as an underdone shoofly pie.

If you think the prosthetic bosom Lawrence hauls around is the phoniest bust in the movie, you must have missed the opening-credit sequence, which finds Lawrence, as FBI agent Malcolm Turner, kicking mass booty in some kind of underground dog-baiting arena for Chinese gamblers. The story rights itself momentarily when it segues into the feds’ quest to track down bank robber/murderer Lester (The Best Man’s sloe-eyed Terrence Dashon Howard), who has stashed $2 million somewhere, possibly in the possession of his former girlfriend, Sherry (Nia Long). Because Sherry may also be his accomplice, Turner and his partner, John (Paul Giamatti), install themselves in a small Georgia town where Sherry’s grandmother, Hattie Mae, lives, and await her arrival. The only problem is that Hattie Mae has just left to visit a hospitalized friend, so with a fancy prosthetic kit and some vocal training, Turner masquerades as the triple scoop of sassy wit and wisdom himself.

The usual complications show up on cue, but it’s all good-natured fun, and the movie doesn’t cop laughs at anyone’s expense. Naturally, Grandma has a boyfriend, a randy old gentleman who drops by at all the wrong hours and, occasionally, in far too few clothes. The local busybodies get suspicious, startling Turner with such accusations as “I know what’s going on over there” and peering through their lace curtains with binoculars. Turner gets in over his head with the impersonation, learning the hard way that Big Momma is the town midwife—who knew kitchen tongs could be so handy? Then there’s an eager-beaver local security guard who stumbles on the ruse and forces the partners to take him into their confidence; he gets to play G-man and stay out of their way, and, in return, is promised a place on the force—well, “You’ll get your F right away,” Turner promises, but the B and I come later.

And then there’s Sherry, a sexy innocent who’s taught her taciturn son, Trent (Jascha Washington), to be close-mouthed about the family’s business. When she speaks wistfully of regret and mistakes, Turner pants in expectation of hearing the whole story. In the film’s showcase scene, Big Momma is called up to testify in church, wherein she gives a rousing speech on the cleansing powers of confession and then, at the prompting of the choir, launches into a pop-and-lock roof-raising hymn.

Lawrence makes the most of his size-XXXL handicap; Turner’s attraction to Sherry may be complicated by the fact that he has to wear a dress, not to mention the suspect-cop thing, but the disguise also affords the unique opportunity to smother her in between the double-stuffs and to watch her prance around the house in skimpy underthings. The laughs are below-the-belt, and so are the pratfall humor and toilet funnies, but Big Momma’s assets—excuse me—are huge. She gets vengeance on the basketball court for little Trent, who’s been thrown off it by the older boys, with big, bumpy hip-checks. She teaches the local biddies to stand up for themselves in a self-defense class run by a carelessly tough karate expert—by the end of it, they all fall on him like a swarm of blue-haired, pie-baking hornets. But most of the fun is just seeing Martin Lawrence in a violently flowered housedress, in elaborate b-ball layers, in horrifyingly body-conscious workout gear, in truss-and-cantilever support garments, and in fat-suit undress, with his tiny head sticking out of the top of a mountain of foam rubber.

The PG-13 rating keeps Lawrence’s trademark pottymouth near the top of the tank—a shame given that his obscene flights of filthy can be Shakespearean in their breadth and depth. And the emotional climax, which finds Sherry angrily calling Malcolm on his “deception”—I mean, really! You’re welcome! I saved you and the kid from your murdering psycho boyfriend—is just silly. But altogether, it’s no stupider than the diamonds-and-virus-and-wheelies-oh-my plot of Mission: Implausible-2. There’s nothing groundbreaking going on here—there isn’t even much that’s particularly bonebreaking (the ending action sequence is all fists and windows, although everyone has a gun)—but Big Momma’s House is brave in one way: It’s a summer movie that dares to go up against the Tom Cruise juggernaut. (According to credible rumors, it was pushed up amid threats of a July release for Eddie Murphy’s multi-Klump turn in the upcoming Nutty Professor sequel.) The soundtrack has it all over M:I-2’s lowbrow headbanging, and, you know, Martin Lawrence in a dress…I’m just saying. CP