(MA15+) **

Director: Jeff Tremaine.

Cast: Johnny Knoxville, Jackson Nicoll.

SO... the Jackass crew have finally made a "real" movie.

This means that instead of just filming themselves being flung about in portaloos filled with poo or doing idiotic things with their genitals, there's a story holding the inane pranks together.

Having said that, the story is still just an excuse for poo jokes and genital-related jocularity, but at least they're trying something new - even if that "something new" dates back to 1948 and the birth of a little TV show called Candid Camera.

Lead Jackass Knoxville, decked out in some impressive prosthetics, has brought back his grandpa character Irving from Jackass 3D.

After the death of Irving's wife (an uncredited Catherine Keener), Irving is "finally free!" but his daughter (Georgina Cates) has other ideas, dumping her son Billy (Nicoll) on Irving while she heads to prison.

Irving's job is to deliver Billy to his waste-of-space father Chuck (Greg Harris) on the other side of the country.

The road trip serves as the skeleton for the sketches which include a malfunctioning children's ride, a drunken bingo session, some shoplifting, and a run in with some well-meaning bikers. All the while, the public is aghast (or in stitches) by what they see, and the hidden cameras capture the real reactions of these real bystanders.

Some of the stunts are admittedly pretty funny. A farting contest between Irving and Billy in a restaurant is genuinely hilarious - in fact, I'm still sniggering about it as I write this - and there are a number of laugh out loud moments, just as there were in the previous Jackass films.

But there is something really unsettling and weird about this film, and I'm not talking about the bit where the little boy does a hilariously wrong striptease to Warrant's Cherry Pie during a girl's beauty pageant.

Where Bad Grandpa doesn't work is, well, as a film. On the one hand, it tells us these are real stunts being witnessed by real people so we can laugh along with their real reactions. On the other hand, it wants us to forget that Knoxville and Nicoll are actors so that we can suspend our disbelief and get wrapped up in their budding relationship.

The film builds towards an emotional note that it can't hit because the entire running time it's switching between saying "we're actors and these are stunts" and "we're characters and this means something". You can't have it both ways. Either we care about a pretend kid and his pretend grandpa becoming friends, or we laugh at the way they've really tricked real people while they're secretly filmed.

The likes of Sacha Baron Cohen's Borat and Bruno never asked us to care about the characters or to make an emotional investment in their journeys - Baron Cohen took a satirical approach that Jackass never strives for. That's the big difference between this and previous "hidden camera" endeavours, and it's what makes Bad Grandpa such a jarring experience compared to Borat's pointed dissection of American life.

This heartwarming approach sits strangely amid its balls-out shenanigans (and I use the term "balls-out" literally here). It's a bold mix (maybe even a ballsy one) but it's not easy to watch as a result.

Unlike other Jackass films, Bad Grandpa is a comparatively mellow affair that appears to be aimed at a slightly broader audience than usual. The stunts are slightly toned down - no one defecates on screen in 3D as in their last effort, thank the gods - but with the emotions ramped up in its kid-and-grandpa storyline, the makers actually run the risk of putting off the fans who followed them through every bone-breaking, fart-smelling moment that came before. They're hear for carnage, not cuddles.

Points to Knoxville though, not only for his not-bad turn as Irving, but for trying something different and for pulling out some genuine belly laughs along the way.

Unfortunately, mixing Candid Camera with a Big Daddy-style story doesn't work.