The first 20 minutes of the cop-reality show comedy "Showtime" are ripe with glossy satire. Robert De Niro plays a no-nonsense Los Angeles detective forced to let TV crews follow him on and off the job so the department won't get sued for his assault on a network cameraman. The guy got in his way during a collar.

The new show's producer (Renee Russo) -- a zealous Hollywood power-broad-in-Prada-shoes with her finger forever on the pulse of the latest demographic data -- quickly realizes her high concept is going to implode if grumpy, frumpy De Niro is all the program has to offer. So she recruits him a wisecracking, showboating, fame-seeking partner (Eddie Murphy) from the dregs of the patrol ranks. She redecorates police headquarters and his dumpy apartment, IKEA-style. She hires former "T.J. Hooker" star William Shatner (in a funnier than usual send-up of himself) to coach her reluctant star on the finer points of eyebrow arching and moving car hood-jumping.

But after spending Act One on all this establishing, director Tom Dey ("Shanghai Noon") utterly abandons the picture's fertile, sarcastic, "real cop" concept and allows "Showtime" to become a high-concept hack job of undiluted Hollywood hypocrisy. Suddenly there's a bleach-blonde Euro-trash bad guy (Pedro Damian) who engineers ridiculously extravagant daylight armored car robberies for no apparent reason except to show off his customized uber-machine gun that fires one-inch ammunition. Suddenly unmotivated cars chases erupt out of nowhere, always ending in slow-mo explosion-crashes.

Murphy's wannabe-actor beat cop discovers an actual passion for real police work, of course. De Niro softens, becomes a loose cannon whose badge and gun are demanded by Captain Cliché. The bad guys are chased through crowds, guns-a-blazin' and hostage-takin', even though there's no credible evidence they were involved in any crime (the cops don't see what we see). And, of course, the TV show is a huge hit, leading to a stunt-heavy climax of absurd proportions.

So much potential goes to waste in this ironically trite "Lethal Weapon" rip-off masquerading as a "Lethal Weapon" parody that the movie is all down hill after the laugh-out-loud first scene, in which De Niro gives a barking, dead-serious, tough-guy speech about what it's really like to be a cop ("I've never had to choose between the blue wire and the green wire...") -- to a class of first graders who are scared stiff of him.

Even in an industry like Hollywood filmmaking, built on insincerity and cheap contrivance, it's pathetically sanctimonious to make a satire that becomes, then grossly exceeds, that which it's meant to mock.