The Titanic director's monstrously-hyped creation does look fantastic but, in trying to cover all the bases with militarist sci-fi, vacuous eco-waffle and an intra-species love story, it's too baggy

Any lingering suspicions that James Cameron has become the Al Gore of Hollywood will be firmly extinguished by his new, monstrously-hyped creation. For a while, it looked like he was giving us a reasonably sweet-natured blockbuster, suggesting that the natural world has, like, the power to heal us all, or something. Then Cameron sends in the helicopter gunships and starts blowing shit up, big time. Way to undermine your own message.

Avatar, for anyone who's had their head in the sand for the last few months, is the first film in over a decade from the man behind Titanic, still the all-time box-office champ. The success of that film presumably allowed Cameron to write his own cheques for this one, and it's a project that's been stewing on the back burner for at least as long, waiting for the special-effects industry to catch up.

And whatever the truth behind the rumoured hundreds of millions spent on it, Cameron certainly gives Hollywood a lot of bang for its buck. Avatar, in all conscience, looks fantastic – a near-seamless melding of fantasy extraterrestrial landscapes and cutting edge computer-generated imagery, all inserted beautifully into the high-testosterone camerawork which Cameron has made his specialty.

But what is this highest-of-high-end image-making aimed at? Cameron has constructed a fable that combines militarist sci-fi, alarmingly vacuous eco-waffle and an intra-species love story that is presumably designed to cover all the bases. The central character is one Jake Sully (Sam Worthington), a paraplegic marine who is assigned to a mining colony on the alien world of Pandora, where he joins a band of nerdy scientists trying to establish friendly relations with the locals; this they hope to achieve by fusing their brains with specially developed beings (the "avatars" of the title) that are a blend of human and alien DNA.

The locals turn out to be spindly blue 10-foot humanoids with distractingly twitchy ears – suggestions that Avatar is somehow channelling Ferngully are not all that wide of the mark. Sully quickly falls for the non-specific mystical rabbitings of the tribe, involving memory-harbouring trees, intimate relationships with flying lizards, and other such prog-rock-influenced stylings. It really is like a Yes album cover come to life.

Sully's position is made considerably more tricky by the genocidal glee of his human military commander, who – in a plot move shamelessly similar to Cameron's earlier film, Aliens – is prepared to cause mass casualties in the service of the sleazy mining-corporation executive.

There are heavy-handed attempts to implant contemporary references (at one point, the marines are told to fight "terror with terror"), but there's no mistaking what Avatar is taking aim at: the founding myth of America, and the incursions of European colonists into indigenous civilisations. The Na'vi, the tribe with whom Sully fetches up, are a sort of grab-bag of generic tribal characteristics – a little bit African, a little bit Amerindian, the equivalent of one of those worldbeat restaurants that serve up teriyaki tortilla and the like.

To his credit, Cameron is a skilful narrative organiser, and fairly soon he has you rooting for the aliens, not those pesky human invaders. (This may not be the most tasteful approach though, to use on an American audience that still doesn't appear to feel especially guilty about what happened to the indigenous people on their own continent.)

Be that as it may, Avatar tries to have it both ways, to be preachy and a thrill-ride at the same time. I can't in all honesty say it pulls it off – it's baggy, longwinded and, for all the light-speed imagery, just not quick on its feet. Cameron used to be the tautest film-maker around, but he just got slack.

• Avatar is released on 17 December