Kick Ass (15)

Matthew Vaughn's cartoonishly violent action comedy takes off from an interesting premise: you cannot dispense justice in a corrupt society if you don't wear the right clobber. It is perhaps a sign of the times that your average teen no longer considers being "a hero" good enough. Nowadays the only thing for it is to be a superhero, and if you can't be one, then act like one. This can-do attitude is what makes the first half-hour of Kick-Ass such a blast.

New York high-schooler Dave Lizewski (Aaron Johnson) is nerdy and needy and knows it. He's so ordinary it hurts: "I didn't have a piercing or an eating disorder," he admits, and the only superpower he possesses is being "invisible to girls". It's a plaint reminiscent of Peter Parker, the regular guy who becomes Spider-Man, except that the goo Dave generates via his wrist isn't the prehensile webbing that allows Spidey to swing through the Manhattan canyons; it's just the stuff that all boys exude at a certain age. True, he's got the background of a would-be superhero, having suffered the loss of a parent (but not to violence) and a slow whittling-away of his self- respect. But Dave's journey of transformation has a more obvious starting-point: given all the people who want to be superheroes, how come no one's tried it?

So he finally unveils himself as "Kick-Ass", walking the streets in a green wetsuit with yellow trim ($99 on the internet) and matching head-mask. This alone might qualify as a sort of courage. His superhero gauntlets are a pair of yellow marigolds, for heaven's sake. No amount of practising in front of the mirror can help, however, when you have to leap across a parapet at a run, and as for the two thugs he confronts during an auto-theft... What's so enjoyable about this probing of the overlap between fantasy and reality is that it posits the questions that might occur to us if we had a sudden urge to play a DIY vigilante. Won't it hurt if I get stabbed? What would happen if a car mowed me down?

Dave finds out, painfully, but he emerges from hospital rebuilt, his limbs larded with metal plates and his nerve endings deadened. "This is awesome," he says. "I look like frickin' Wolverine!" Kick-Ass 2.0 is ready to go back to work. The blood-boltered caper that follows has its moments, but you will notice how the script (by Matthew Vaughn and Jane Goldman) becomes more conventional – more like other superhero movies – as it proceeds. Dave's confessions of inadequacy at the start, and his inchoate grappling with the idea of self-transformation, are by far the funniest and best-written parts of the film.

That said, the real crowd-pleasers are two other self-styled caped crusaders, a domesticated Batman-and-Robin duo known as Big Daddy (Nicolas Cage) and his daughter Hit Girl (Chloe Grace Moretz). Our first sight of them is at target-practice: Big Daddy is pointing the gun, and Hit Girl is his target. The walls of their den are studded with every sort of automatic weapon. "Weird" and "mad" just about cover this partnership. You don't give much for the chances of their arch-enemy Frank D'Amico (Mark Strong), especially after the inventively lethal fashion in which Hit Girl dispatches a roomful of his goons. The violence is at about the same point on the dial as Sin City and Watchmen, though the BBFC has for some reason decided that Kick-Ass merits a "15" rather than an "18". (It's based on the comic book by Mark Millar and John S Romita Jnr). Perhaps they reasoned that the shooting and skewering and slicing are so extravagant that nobody could really take it seriously. They may be right. But you don't have to be a Daily Mail reader to feel a ripple of offence at one particularly gratuitous use of a – or rather the – four-letter word. I have no problem with swearing on screen – most of the time I barely even notice it. But I do wonder how it contributes to anything – fun, credibility, the tattered remains of our innocence – to have an 11-year-old girl address a bunch of men she's never met before as "c**ts".

Vaughn's other, less serious breach of taste is his repeated shot of a gigantic billboard featuring Claudia Schiffer – his wife – as photographed by arch celebrity-fawner Mario Testino. It just says "no class". He makes happier decisions in the casting of the film. Aaron Johnson has gone from heartbreaker in Nowhere Boy to bonebreaker in this, and does the dreamy American teen very convincingly. Even better is Chloe Grace Moretz as Hit Girl, a cherubic death-dealer in a purple Clara Bow wig. (I look forward to seeing her in the US remake of Let the Right One In). Even Cage, who's been phoning them in for a while, strikes some unexpected notes as Big Daddy, wearing the specs and wispy moustache of a geography teacher but apparently as ruthless as a Mossad agent.

You get the feeling that Kick-Ass could be a huge "audience movie", given its covering of certain bases – teenage angst, parent-child oddity, comic-book violence – and the sulphurous energy driving it along. It threatens to be a more interesting proposition, as a sly commentary on superhero movies, than it eventually proves, and I'm not sure I can forgive it that unwarranted obscenity. But it does, indubitably, kick ass.