Though "CB4" starts out as a fairly amusing rap variation of "This is Spinal Tap!" it quickly sinks into a raunchy, sexist series of off-the-wall gags that go nowhere.

"Saturday Night Live's" Chris Rock co-produced, co-wrote and stars in "CB4" as Albert, a middle-class Los Angeles youth who, with a pair of friends, Euripides (Allen Payne) and Otis (Deezer D), forms a rap group. In one of the film's earliest — and funniest — moments, we see in their various incarnations (including the "Bag Heads") before the trio finally settles on a "gangsta rap" pose.

Inspired by a local hoodlum named Gusto (Charlie Murphy, Eddie's younger brother), they form "CB4" (named after Gusto's prison accommodations, "Cell Block 4") and adopt as stage names MC Gusto, Dead Mike and Stab Master Arson.

The film begins with videographer A. White (Chris Elliott) showing a few minutes of his in-progress documentary of the group, then has him following Albert around as he tells the story of how it all came together, which we see in flashback.

Naturally, it winds up with Gusto escaping prison and going after CB4, complete with car chases and shootouts.

There are some inspired moments along the way, including a Spike Lee parody, goofy pseudo-music videos and amusing cameo appearances by the likes of Ice T, Ice Cube, Halle Berry, Flavor Fav, etc.

But the pacing is dull and the jokes start to sag early on, recovering only infrequently, which is too bad since most of the cast has the energy and talent to carry it off. (Especially Khandi Alexander, as a sexy groupie.)

Unfortunately, director Tamra Davis doesn't quite seem to have a clue as to how comedy works, and so much of this is so incredibly misogynistic that it's hard to believe a woman was behind the camera.

There are also subplots that are woefully underdeveloped, from a self-righteous politician ("Saturday Night Live's" Phil Hartman) who has CB4 arrested to Albert's jealous and indignant girlfriend (Rachel True).

Actually, the central idea here — that a few suburban kids who just want fame and money are feigning anger to sell records — is a pretty good start for a piercing satire. But "CB4" goes instead for silly sight gags, vulgar jokes and feels more like a stretched-out TV skit than a movie.

"CB4" is rated R for considerably profanity and vulgarity, along with sex, nudity, violence and drugs.