Drunk on pizzazz and primary colours, Sam Raimi's first two Spider-Man pictures went some way towards persuading filmgoers that the successful comic-book adaptation did not have to be an exercise in low-cal existentialism. Peter Parker, the teenage nerd who gained the powers of a god after being bitten by a radioactive spider, may never have got over his indirect role in the death of his uncle, but that didn't mean he had to spend his days skulking in damp caves like that peculiar millionaire from Gotham City.

Whereas Tim Burton brought the solemnity of Ingmar Bergman to Batman, Raimi's gloriously entertaining Spidey movies - with Guillermo del Toro's Hellboy, the best ever superhero flicks - crackled and buzzed with the same class of energy that illuminated Singin' in the Rain and Dumbo.

That optimistic vitality is on triumphant display in the first 20 minutes of Spider-Man 3. Moving, once again, through a New York that, though nominally contemporary, doesn't seem to have escaped the Kennedy presidency, the picture finds Tobey Maguire's Parker blissfully content with work, love, and the rewarding trials of superheroics. Mary Jane (Kirstin Dunst), his squeaky girlfriend, has landed a role in the sort of staircase-heavy Broadway show not seen since Ethel Merman was a star. And Spider-Man, once vilified, has been adopted as a mascot by the city's tourism authority. Lying together on a great web, gazing at twinkling constellations, Mary Jane and Peter appear destined to live their lives in a shared storybook idyll.

Their bliss is, however, short-lived. The sombre posters have, somewhat worryingly, already alerted us that this is to be (oh, not again) a darker superhero romp.

While the two lovers are distracted, a meteorite lands behind them and emits a sinister inky entity. Later, the being will blacken Spider-Man's suit and amplify its wearer's tendencies towards arrogance and insensitivity. Will we be stuck with a Bergman Spidey after all?

Happily, no. The scenes following Peter's passage into hubristic egotism - more Saturday Night Fever than Seventh Seal - are among the funniest in the entire series. Adopting the swaggering gait of a juiced-up cockerel, he flings himself into dance moves and flashes his teeth with an insane confidence that sits comically against Maguire's reliably sat-upon demeanour.

If we were allowed the room to enjoy Parker's transformation, then Spider-Man 3 might, indeed, prove a worthy successor to the first two episodes. But Raimi, perhaps aware that this will likely be his last outing at the helm, has made it his business to pack in sufficient beloved elements of the original comic book to fuel a dozen features.

Not only do we encounter the darker Spider-Man, but we also get to see Eddie Brock, a professional rival of Peter's, played well by Topher Grace, don the black suit and become the savagely malicious Venom. Thomas Haden Church essays the role of Sandman, a shape- shifting villain with the capacity to drift about the city in granules. James Franco is on hand to continue the evil work his father, the late Green Goblin, began with those colourful grenades and that suave flying tea tray.

Meanwhile, Gwen Stacy - once the personification of early 1970s grooviness, now the icily blonde Bryce Dallas Howard - floats about the place, ready to cause friction between Mary Jane and Peter.

There is, even for a film that stretches to 140 minutes, far too much going on here. Sandman's transformations are spooky, and the slavering, toothy Venom is almost as unnerving as he is in the comics. And, yes, the rapidly accumulating characters are brilliantly cast. But with all these plots jostling noisily against one another, the picture never finds sufficient space to satisfactorily develop any of the stories.

That said, Spider-Man 3 still exudes just enough magic to justify its existence (if not its running time). Barring miracles, a fourth episode would, however, almost certainly constitute a step too far.

Marvel creator Stan Lee, who appears here in a cameo, might well sum it up in his favourite two syllables: 'Nuff said.