Hollywood's treatment of mental disability has tended to consist of the likes of Sean Penn or Dustin Hoffman method mugging furiously for an hour or so.

So it's nice to be able to welcome a major film where key characters are actually played by the mentally challenged...as well as Johnny Knoxville.

He's Steve Barker, a kind-hearted cove who employs his sacked company janitor only for him to lose his fingers in a lawnmower incident.

Faced with a $28,000 medical bill, his dodgy uncle Gary (Cox) encourages him to pose as a mentally disabled athlete and pit himself against pentathlete Jimmy (Flowers) in the Special Olympics.

Steve duly feigns it only for his fellow competitors to twig his pretence...but stick with him just to see Jimmy beat.

Grimly determined to challenge the politically correct community that labels people "special", this would have a much stronger suit if it was genuinely funny.

However, it merely places mentally-challenged actors in a comedy that bears a close relation to Date Movie in the awfulness stakes.

Director Barry W Blaustein crudely conveys a message of inclusiveness and and tolerance via a plot that strives desperately to be both naughty and nice.

One of the few instances of genuine moral outrage is when Barker is lamped by a furious priest during confession and hurled down the church steps.

Elsewhere, Blaustein pussyfoots around his subject, rendering his mentally challenged cast a mildy amusing backdrop to Barker's romantic exploits with Katherine Heigl's tasty coach.

It's certainly a step in the right direction - the Special Olympics have welcomed it as the breakthrough of a lingering taboo - but the comedy never lives up to the central conceit.

Tim Evans