THERE ARE MILD SPOILERS AHEAD

Okay, breathe. Avatar is easily one of the most anticipated films of the year - and rightly so. It is a technological marvel, and a return to sci-fi filmmaking from a director that many of the writers, editors and readers of this site consider to be some sort of Cinematic Father Figure - a peddler of pure, imaginative wonderstuff. With James Cameron involved, there is a hope that this will offer something more nourishing, something different from the blockbuster geek fare that we have seen of late. And that's certainly true. So with my spoiler hat firmly on, we're going to wrestle with this huge, beautiful, radiant beast called Avatar.

What is surprising is how Cameron is grappling with many of the iconic aspects of his previous films, namely Aliens. Within the opening hour of Avatar, you've already been presented with gruff marines, dropships, mining colonies, slimy men in suits, hypersleep pods, big metallic walkers, bio labs and an amoral, buck-chasing corporation called The Company. However, whereas Aliens was a much more straightforward sci-fi action flick in scope, Avatar is self-consciously ambitious, aiming for the giddy heights of epic fiction.

The story is set on Pandora, a lush planet that is home to an indigenous humanoid species of ten foot tall, blue creatures called the Na'Vi. The relationship between the nature-worshipping, tribal Na'Vi and the industrialist, expansionist humans is fraught. A number of scientists, led by botanist Dr. Grace Augustine (Signourney Weaver), attempt to research and communicate with the tribes, using Avatars, an approximation of the Na'Vi's biological structure, which allow human 'pilots' to breathe the hazardous Pandoran atmosphere.

Into this melting pot comes wheelchair-bound ex-marine Jake Sully (Sam Worthington). Ostensibly, he's assigned to Augustine's team, but other sectors of the human colonial force are eager for a piece of him. Terse marine Colonel Quaritch (Stephen Lang) and project administrator, and Yuppie-of-the-Future, Parker Selfridge (Giovanni Ribisi) both want him to act as a mole, relaying important information about the Na'Vi society for tactical gain.

So, once again we find ourselves at the cross-roads between science, business, the military and humanity. Sully, a soldier at heart, doesn't think twice before accepting Quaritch's mission. But before long, he is torn, as he finds himself freed of his infirmities in Avatar form - shown in a breathless sequence as Av-Sully runs through a Pandoran field, curling his toes in the dirt.

It is testament to the grace of the Avatar's effects that such small moments work. Cameron, aided by VFX teams from WETA and ILM (and a handy couple hundred million dollars), has created a film-world that is seamless and immersive. While maybe not exactly a paradigm shift, it is a startling culmination and refinement of the VFX innovations that have been played with over the last decade by directors like Peter Jackson and Robert Zemeckis.

Avatar has the edge over the likes of Lord of the Rings, King Kong and Beowulf, not simply due to its over-inflated budget, but because of the inspired amount of design work that has gone into fleshing out Pandora's flora, fauna and geology. As Sully explores the depths of the jungle, each frame bursts with creative energy that is made all the more vivid by the texture and depth presented by the 3D visuals. Individual animals, plants and locations prove to be immediately striking, yet memorable - from vibrant, ethereal flying lizards to floating mountains and tree barks that, in the night, light up to the touch. It is a world for the audience - like Sully - to get lost in, a cinematic experience that simply cannot be matched by home entertainment systems.

This is pure spectacle. It needs to be enjoyed on this level, since once Jake (literally) stumbles into Na'Vi society, Avatar reveals its under-developed, unsophisticated narrative core. The natives are depicted as a hodge-podge of Native American and African tribespeople, and the evocation of the exploitation of such indigenous populations by imperial powers throughout history is blunt and unsubtle. The conflict between the humans and the Na'Vi is shot through with nods to Vietnam and - to lay it on extra thick - Iraq, as it is revealed that the hub of their society happens to be on top of a rich source of 'Unobtanium', a mysterious substance that is of great value to The Company. Throughout, Avatar poses itself as having Something To Say, and does so with the heavy-handedness of a drunk with a megaphone. There's a chance that such an approach could work, but the thematics of its narrative are outmoded and a little backward.

Much has already been made of the Dances With Wolves aspects of the plot, as Jake, sighing under his White Man's Burden, slowly becomes seduced by the native society, gains their trust over time and, eventually, leads them against the Evil Humans. This is a popular story model for colonial cultures wishing to redress historical atrocity - check The Last of the Mohicans, or plenty of other Hollywood movies that don't simply characterise Native characters as savage scalpers - because it provides the (assumed) white audience with an empathetic lead to follow into the foreign culture.

Avatar makes a handful of awkward decisions that will make Post-Colonial Theory-savvy intellectuals sigh with the hollow joy of smug elitism. Strike one: Cameron weighs too heavily on exoticism, with the mystical properties of the Na'Vi's (naturally pantheistic) religion being overstated at every turn (to the point of a literal deus ex machina late on in the film, and some slightly cringe-worthy religious ritual sequences). Strike two: the film ties together its love of Na'Vi society with Sully's romance with princess Neytiri (Zoe Saldana), a narrative trick that is hackneyed and predictable, not to mention a little misogynistic in its implications.

Strike three, the worst of the bunch: the Na'Vi just aren't as interesting as the humans. For the most part, they are stock characters - 'noble savages' that are quick to anger, alien in their values and emotionally distant. While the motion capture technology works well to recreate the minutiae of both Saldana and Worthington's performances, they are swept aside by their non-CG companions. Ribisi, Lang and Weaver - and to a certain extent Michelle Rodriguez, as kick-ass chopper pilot Trudy Chacon - are all remarkable.

And while they all act superbly, their characters are given nuance and shape by ideas from the scripting stage, with minor touches that are economical in their eloquence. Colonel Quaritch is a scarred, mean bastard who gets all the pumped up 'hoo-ah!' lines of the sort that are still quoted by Aliens fans, but his dedication and drive is shown by his mad propensity for rushing out into the Pandoran atmosphere without an oxygen mask, holding his breath in a display of true grit.

Likewise, Parker is an even slimier version of Aliens' Burke - the kind of guy that practises his putting in between planning his next act of genocide - and Augustine is a guarded, chain-smoking scientist, whose personality blooms when out and about in Avatar form. None of the Na'Vi have this sense of character: you've seen them before, they're archetypes. So while we're seduced by their world, our empathy and interest is still located elsewhere.

I overheard a chap yesterday say 'the script is the cheapest part of a film', so it is a shame that Avatar doesn't boast a narrative sophistication to match its visual splendour. That said, its story is there, and does a good job at propelling the film forward, setting up binaries of good/evil, and even planting seeds of emotional attachment if one is so inclined (even if the most potent of these, Jake's disabled narrative, is hardly explored). It's only because it runs the risk of trying to do too much that it inevitably feels lop-sided or confused in its intentions - is it escapism, is it didactic, is it romantic, "feel-good" or cautionary?

I suppose most people won't be bothered, and they have every right to be blown away by Avatar. It's an event movie worthy of the description. It is packed with the kind of 'wow' moments that blockbusters seldom indulge in any more (coming off a little like Jurassic Park in the process), and the closing hour is defined by a stunning action sequence that is impressive in scale and deft in its direction. It is every bit the technical achievement that Cameron promised it would be. However, there is no dodging its weak, underwhelming story. And I don't go back to rewatch The Terminator, Aliens or T2 time after time for the special effects.