Zack and Miri Make a Porno, Kevin Smith, 101 mins, 18

In the new comedy from writer-director Kevin Smith, the Zack of the title is Seth Rogen, playing much the same cheery, out-of-shape slacker as he did in Knocked Up, only without the pot-smoking. The Miri of the title is Elizabeth Banks, who's considerably less out of shape, but no less cheery.

They're best friends who have shared an apartment since they were at high school, but have never contemplated sleeping together. They both have jobs, but for some unknown reason they suddenly find that they can't afford to pay their rent, so Rogen has a brainwave: they'll borrow some cash from their friends and family. Actually, no, that's not his brainwave. His brainwave is that they'll video themselves having sex, and then sell the tape to everyone they went to school with. For some other unknown reason, they persuade several acquaintances and professional strippers to join in. The question is, can they get physical without compromising their friendship?

I would have thought that producing a DIY porn movie might have more troubling ramifications than making you realise you fancy your flatmate. But Zack and Miri Make a Porno goes straight down a well-worn romantic comedy route, however filthy the dialogue might be. Nude scenes and four-letter words notwithstanding, it's got less gritty realism than the average Doris Day vehicle. For instance, Rogen is only semi-competent as a porn director, and yet Banks starts cooing as if she's seeing a whole new artistic side to him, which just goes to show how standards are slipping. In Knocked Up, he had to get a day job and his own apartment before an exquisite blonde fell for him. Here, all he has to do is point a video camera at a pole dancer, and he's surrounded by new friends, thanking him for opening "a world of possibilities". Yep, sex in front of strangers now counts as "a world of possibilities".

It's still one of Smith's better films. Instead of typing four-page monologues and then getting his New Jersey buddies to read them out, as he usually does, he's written a warm, cosy script, and hired two of American comedy's brightest young stars. Banks, especially, proves that she's the most effervescent romantic female comedian since Meg Ryan drove home from college with Billy Crystal. But her megawatt radiance highlights some of the screenplay's inadequacies. It's impossible to believe she'd be single, impossible to believe that the promiscuous Rogen wouldn't have made a move already, and impossible to believe that she'd volunteer to get naked with him for all the world to see. I'm sure there are porn films with more plausible stories.

If Smith had included some more jokes, then the by-the-numbers plotting might not have mattered, but the humour begins and ends with the dialogue's gobsmacking rudeness (which can, admittedly, be quite funny). I'd call it a clichéd rom-com, except that it's not as challenging as that. It's a twinkly, let's-do-the-show-right-here fantasy: High School Musical with anal sex.They're best friends who have shared an apartment since they were at high school, but have never contemplated sleeping together. They both have jobs, but for some unknown reason they suddenly find that they can't afford to pay their rent, so Rogen has a brainwave: they'll borrow some cash from their friends and family. Actually, no, that's not his brainwave. His brainwave is that they'll video themselves having sex, and then sell the tape to everyone they went to school with. For some other unknown reason, they persuade several acquaintances and professional strippers to join in. The question is, can they get physical without compromising their friendship?