There seems to be an unwritten rule that movies starring ex-stand-up comedians must come to a grinding halt at some point for the star to have a vanity improv scene.

Every Robin Williams has such moments -- even his syrupy, sentimental pictures. Every Martin Lawrence movie does too. In "Blue Streak," the improv moment comes when Lawrence dons a nappy pigtails wig, gnarly false teeth, body padding and a velour jogging suit to pose as a hyperactive pizza delivery boy.

For that one scene, any common sense regarding the story is put on pause and Lawrence cuts loose with an epileptic booty bump dance and a lot of babbling smack, all of which is designed to produce seat-bouncing laughs (it doesn't), but has little to do with the movie.

Let that scene slide however, and "Blue Streak" is a surprisingly consistent gut-buster, in which Lawrence plays a jewel thief forced to pose as a cop in order to retrieve a precious diamond from his last heist.

The movie opens with a safe-cracking scene that ends badly for our he-so-crazy hero when one of his partners gets greedy and pulls a gun on him. With the alarm tripped and the law quickly surrounding them, Lawrence ducks into a construction site and secures his loot -- a plum-sized diamond -- inside an air duct just before being arrested.

Release from jail two years later, he returns to the site only to discover it's become a police station and throws a hilarious curb-side conniption fit before composing himself and walking away, gears a-grinding on ways to get inside without arousing suspicion.

That's where the pizza delivery schtick comes in, and when it fails to get him beyond the desk clerk, he pick-pockets a security pass off a plainclothes cop, forges a stellar arrest record, takes a crash course in police procedure by watching one episode of "COPS," then waltzes into the precinct, pretending to be a transferring detective so he can get inside for a few hours and crack the ventilation system.

Of course, nothing is that easy, so before long Lawrence finds himself, ironically, on the burglary beat, assigned a greenhorn partner (Luke Wilson, "Home Fries"), hot on the trail of heroin smugglers, and mistaken for Internal Affairs mole and for an FBI plant.

This ridiculous progression of events is surprisingly light on significant loopholes and frequently funny as the crook's criminal mind turns him into an ace crime fighter while going to lunatic lengths to get his diamond back.

Sporting his standard "daaamn, baby!" persona, Lawrence isn't required to stretch much here, but the script is a good fit, affording him plenty of chances to season the movie with his comic talent, making some of the funniest moments small laughs (Lawrence getting into the back of his police car by force of habit) instead of exaggerated set pieces.

While the story progress in "Blue Streak" is often predicated on the weakest of plot points, director Les Mayfield ("Flubber") corrals the loose ends and allows his jocose cast the freedom they need to make the best of every scene.

In addition to Lawrence and Wilson, who have a good anti-buddy rapport, comedian Dave Chappelle ("200 Cigarettes") steals a few scenes as Lawrence's former partner, arrested for petty theft, who threatens to blow his cover.

This picture has more than a few shopworn conventions to overcome (is there any building anywhere in the real world where air ducts flip open with little more than a prying fingernail?) and its shootout-explosion-chase-stunt finale is pretty standard fare, but none of its shortcomings ever stop "Blue Streak" from being fun.