Spider-Man

Along came a spider

English summers and blockbusters seem to go together – both of them, as Terry-Thomas would have said, an absolute shower. But while the weather's playing true to form, the biggest event movie of the season has its share of sunny spells. Spider-Man begins so promisingly that it looks less like a comic-book movie than an offbeat comedy of metamorphosis. For at least 45 minutes it exhibits something I could never have imagined of a film with a $100m-plus price tag: in a word, charm.

The reports go that director Sam Raimi, who launched his career as a horror-meister, insisted on the unconventional casting of Tobey Maguire in the title role. He was right to. Maguire's oddly innocent, sleepy-voiced persona has cast a spell in movies both good (Pleasantville, The Ice Storm) and bad (The Cider House Rules), and now in this dual role he gives Spider-Man a complexity that a more straightforward leading hunk would have missed. When we first see him he is high-school nerd Peter Parker, bespectacled, bullied and burning with a secret, hopeless love for the girl next door, Mary Jane (Kirsten Dunst, in redhead mode). "This, like any story worth telling, is about a girl," says Peter, and Dunst proves a lovely match for Maguire: it might be something to do with the fact that both of them have terrific smiles, his slow and guileless, hers maybe the kindest in movies.

The script, by David Koepp, takes its time in the early scenes, allowing us to savour Peter's anxiety as a loner and dreamer in the modest house he shares with his aunt (Rosemary Harris) and uncle (Cliff Robertson). His life begins to change when a genetically modified spider nips him on the hand during a school science trip; pale and woozy, he later collapses in his bedroom and wakes up the next morning mysteriously enhanced. His weedy frame is now fronted by rippling pecs, his vision so crisp that he has to discard his spectacles – he seems more alert, more alive, rather like Jack Nicholson did after he got a bite of something nasty in Wolf. He's hormonally charged, too, as a quick peek down his boxer shorts indicates, and Raimi has some lascivious fun showing Peter's great new discovery: he can throw long looping threads of viscous matter off his wrist. That mightn't be news to any teenage boy, but this stuff happens to be spider's webbing, just the thing to help you swing through the canyons of Manhattan like Tarzan on his jungle vines.

Meanwhile, another metamorphosis echoes and parallels this one. Norman Osborn (Willem Dafoe) is a nutty millionaire scientist and father of Peter's best and only friend, Harry (James Franco). After an experiment in the lab goes wrong, Osborn creates his own Mr Hyde, a monstrous alter ego named the Green Goblin, whose cackle is only slightly less camp than that of Kenneth Williams in Carry On Screaming. His first public appearance, flying low on a sort of aerial surfboard amid the skyscrapers, is eerily reminiscent of the suicide jets of 11 September – the film underwent strenuous editing in the wake of the attacks. It also marks the point at which the quirky comedy begins to flag and the routine procedures of the blockbuster take hold. This is not to blame Dafoe, whose gaunt, angular features have always made him look less like a movie star than an Egon Schiele hunger victim. His one great scene here is a dialogue with his tormenting inner voice; when he asks the demon to show himself, he is pointed towards a mirror and the horrifying truth of his own infected soul. His adopting the leering mask of the Goblin, however, is a mistake: Dafoe's face needs no props to give us a fright, as anyone who saw his toothy bloodsucker in Shadow of the Vampire will attest.

The odd thing is, the more ambitious the action sequences become, the less convincing do they seem. When Peter first discovers his preternaturally swift reflexes, he uses them to best the school bully – and it's perfectly enjoyable. Then, tentatively entering himself in a wrestling competition, he KOs a muscled champ named "Bone-Saw". Three more cheers for Pete.

By this point, his uncle senses that the boy is "changing" (he doesn't know the half of it) and gently warns him that "with great power comes great responsibility". Yes, and with a huge budget comes a responsibility not to let computers run the show. Once Spider-Man begins leaping over the rooftops and swinging through rush-hour traffic, one loses a sense of his agility, of a human form suddenly overmastering gravity. However hard the stuntmen work, the acrobatics look strangely weightless and cartoonish, and the prospect of danger becomes no more urgent than that of a kid's video game. Will he rush into the burning building to rescue a stranded child? Or save a dangling cable car from plummeting to its doom? Go on, take a wild guess.

One suspects that Sam Raimi himself is much less interested in the computerised whizz-bangs than the burgeoning intensity of the scenes between Peter and Mary Jane, who's not quite sure whether she's falling for the boy next door or a weirdo in a webbed ski-mask: "I think I have a superhero stalker," she says to Spidey after he has saved her from an assault. "I was in the neighbourhood," he replies suavely. Yet just when their relationship is about to swoon into romance, Peter, the lovesick swain, does the unthinkable and backs off, talking instead about how he'll always be there for her "as a friend".

What?! Has Peter suddenly begun sprouting unsightly body hair or an extra set of arachnoid limbs? Does he secretly prefer chasing flies? No, nothing so outlandish, I'm afraid. His gentlemanly withdrawal can be put down to the studio's love of a sequel – they're going to spin out this web for as long as they can.