It's always a pleasure to see an animated movie that eschews the trappings of shopworn formulas kiddie flicks, and the capriciously clever, clay-rendered "Chicken Run" is nothing if not unique.

A goof on "The Great Escape" and "Stalag 17," but set in a chicken coop kept by a tyrannical, hairpinned harpy of a farmer's wife, the picture serves up an charming self-confidence fable, refreshingly short on stock cartoony characters (no orphans or cutesy simian sidekicks) and long on the distinctive, malleable, stop-motion genius of its director, Wallace and Gromit creator Nick Park.

Park worked hand-in-hand with Peter Lord, best known for Peter Gabriel's "Sledgehammer" video, but the film's winning visual style is unmistakably Park's. Every character is remarkably alive with verve and personality, thanks largely to his trademarks: Ridiculously wide mouths with abbreviated rows of teeth, remarkably expressive, oversized eyeballs, ledge-like foreheads and the tendency for all his characters to walk with a little waddle.

The story centers around a daydreaming hen named Ginger (voice of Julia Sawalha, Saffron from "Absolutely Fabulous"), so determined to literally fly the coop that she's forever concocting failed escape attempts that land her in "the hole" -- a coal hopper along side the farmer's house.

When the greedy farm wife (animated with a nasty scowl and malignantly voiced by Miranda Richardson) becomes irked by low egg production, she invests in a gigantic, menacing Rube Goldberg machine that is fed chickens in one end and pops chicken pies out the other -- a development that has every chicken in the coop clucking to Ginger for a workable getaway plan.

Enter a roguish rooster named Rocky (Mel Gibson), who literally falls from the sky and charms the hen house inhabitants into believing he can teach them to fly out of there -- if only he hadn't hurt his wing when he landed in their yard, darn it.

Hunky, cocky Rocky is really on the run from a circus act and only landed in their yard because he was shot out of a cannon as part of the show. They're somehow not aware that roosters can't fly anymore than hens, so Rocky plays along. After all, he's not about to disappoint a flock of cute chicks showing him with hero worship -- at least not until he absolutely has to.

"Chicken Run" is undeniably the current zenith of the claymation art form, and while it's entertaining and certainly as original as any of Park's short films, the picture suffers some for being stretched to feature length from only about 50 minutes of good material.

Park and Lord find quality comedy in the confounded farmer (voice of Tony Haygarth), afraid he's going mad when he starts to notice the chickens organizing against him. Park's deft wit is also visible in the supporting characters, like the two shady rats (Timothy Spall and Phil Daniels) who supply their feathered friends with spoons (read: shovels) and other tools to aid in their escape. However, the chickens eventually fix on a scheme to build a makeshift airplane (silly, yes, but it works in context), leading to a hilarious last reel that makes up for many of the movie's slight shortcomings.

Gibson's goofball charisma is perfect for Rocky the rooster and Sawalha's obstinate incantations lend the heroine a perfect mix of resolution and apprehension that she must overcome. In fact, all the voice actors give their characters memorable personalities to match their toy-ready looks.

The filmmakers can even be admired for their calculated handling of the inescapable prison camp illusions. There's a dark, weighty undertone that plays subtly beneath many scenes, even when "Chicken Run" is at its funniest.

But the movie is slow-paced and packed with filler just to distend it to a mere 85 minutes. "Chicken Run" is ingenious, cutting-edge animation with a sharp story that will make for a great home video. In that format it will absolutely stand up to multiple viewings. But as a theatrical release, it's only matinee material -- fun and amazingly rendered, but ultimately a little flat.