Attack the Block, Joe Cornish, 88 mins (15)

The only people who can stop the extra-terrestrial advance are a gang of south London hoodies in this back-to-basics home-grown sci-fi spoof

Joe Cornish's Attack the Block – about hoodies fighting aliens in south London – is pretty snazzy stuff for someone who, as half of TV and radio comedy duo Adam and Joe, started out making film spoofs with woolly toys.

In a sense, Cornish hasn't got much more sophisticated than that. When the first alien appears in this film, you can actually – ever so slightly – see its luminous green rubber fangs wobble. Attack the Block's creatures aren't CGI, but contain people running on all fours. That's the sort of homemade touch that gives Cornish's film its endearing appeal.

Attack the Block has an equally no-frills premise: an extra-terrestrial crash-lands in south London, just round the corner from Oval Tube station, and falls foul of a gang of teenage hoodies, who then reap the consequences of their act when hordes more aliens follow. When we first see the gang mug a young woman and then, for the hell of it, appallingly mistreat the lone alien, we feel we're seeing human nature at its worst – with the film playing on our entrenched anxieties about black inner-city youth. But once more aliens drop from the skies causing spectacular chaos, the gang rise to the occasion, teaming up with their muggee (Jodie Whittaker). Some of them even make it to a redemptive ending – especially their leader Moses, played by the charismatically tough John Boyega, leading a more-than-game cast of young newcomers.

The film comes across as a grittier, marginally less spoofy Afro-Caribbean answer to Shaun of the Dead, whose director Edgar Wright executive-produced this. Like Wright, Cornish is a hardcore genre buff – British science-fiction greats are paid homage in place-names such as Wyndham Tower and Ballard Street – and I'll bet that he has placed a million and one precise movie nods that escape me. But if there's a film-maker that you can tell Cornish has studied thoroughly, that would be John Carpenter, whose masterly way with a siege is reflected in a taut sequence with Boyega and Whittaker trapped in a police van while creatures wreak bloody havoc outside.

The film alternates suspense with flashes of mild gore gross-out and cheerful farce: there's a nice scene in which a bunch of girls set about the aliens with roller skates and standard lamps (the girls, in fact, are feistier and mouthier than the boys, and underused in this somewhat male affair). But Cornish runs out of fresh ideas too soon, and the film finally feels like a string of stand-alone moments that don't quite build up to anything larger.

Everything finally turns on Moses's redemption, and there's a moment of genuine poignancy when this menacing outlaw figure is revealed to be a lonely, disadvantaged 15-year-old. One of the script's more daring moments is his speech about how the invaders are perhaps the system's way to put the black population down: it sort of works, sort of doesn't, because it comes across as at once heartfelt, angry, creaky, quite possibly more of a piss-take than it sounds, and (I suspect) a knowing movie reference that I'll kick myself for not getting.

The film's not quite exciting enough for pure action appeal, nor funny enough really to compare with Shaun – although that film's Nick Frost contributes his familiar affable sleazeball role as a drug dealer. And Cornish – himself white, middle-class and no longer in the first flush of streetwise youth – cheerfully mocks his own claim to authenticity in the form of a wet posh boy (Luke Treadaway) tagging along for a bit of reflected cred.

But the young cast of unknowns – including likeable Leeon Jones and the winningly nervy Alex Esmail – have the ring of plausibility both in their strut and their language. It's the slang that's one of the film's real selling points ("You wanna Merc me?", "Alien invasion London wide for real", that kind of thing). One of the amusing entertainments of the week will be reading middle-aged film critics trying to catch the argot in their reviews. In which case – allow it bruv, allow it.

Next Week:

Jonathan Romney dives further into Cannes, and watches new films from Terrence Malick, Pedro Almodovar, Lars von Trier...you name 'em

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