The Last Airbender, M Night Shyamalan, 103 mins (PG)

The Asian-American director M Night Shyamalan must be getting disenchanted about his children's taste in stories.

His 2006 film Lady in the Water was originally concocted as a tale with which to beguile his children at bedtime, presumably with a high approval rating from them. The story of a naked, draggle-haired "narf", or quasi-mermaid, who lures Paul Giamatti to her nasty undersea home was a critical and box-office disaster.

The Shyamalan children presumably played little part in their father's next film, The Happening (2008), about a ghastly virus that makes American people throw themselves off high buildings. But some time in the past two years, his daughters told M Night how much they loved the Nickelodeon TV cartoon Avatar: The Last Airbender – a huge success in 120 countries. He watched it with them, and decided it was a natural for movie treatment.

It isn't. Perhaps it might have been, had someone other than M Night written it, or had any good actors been available to appear in it, or had a decision been made not to shoehorn crappy 3D effects into it, or had some script overseer wondered how viewers would cope with inexplicable jumps from one scene to another, and had some kind and wise friend of the producer explained that a lot of mystic posing does not an action movie make.

Sadly, nobody was around, so Mr Shyamalan, as writer, producer and director, takes full responsibility for this vast, freakish and gobbling $150m turkey. The story is an ambitious conflation of Wagnerian/Middle Earth sagas with the X-Men, a lot of Zen stuff about reincarnated super-beings and a touch of Harry Potter. It's set in a world of four elemental nations – Earth, Air, Fire and Water – in which the nasty, colonialist Fire Nation is trying to dominate its kindly, herbivorous neighbours. In every nation, there's a samurai-style class of warriors who can "bend" their home element to good effect, and fight the invaders. But the Fire Nation has already killed every Airbender (or so it thinks) and is trying to wipe out the Waterbenders, then the Earth-flingers.

One day, our young waterbender heroes, 14-year-old Katara (Nicola Peltz) and her lummoxy brother Sokka (Jackson Rathbone), release from an icy grave a bald youth called Aang (Noah Ringer). He turns out to be not only the last remaining Airbender but also the Avatar: a mystical being like a midget Dalai Lama, who can pull all the nations together in harmony. My God, the excitement. Our youthful trio travels from South Pole to North, trying to rouse the enervated populace, while, on a wonderfully clapped-out, spiky rust-bucket of a ship, the Fire Nation's commander Zhao plots world domination and sighs over his doltish son Prince Zuko, who has been banished until he tracks down the Avatar.

Zuko is played by Dev Patel, the nervy young star of Slumdog Millionaire. Possibly worried (with reason) that he lacks tough-guy qualities, he spends the film with his face locked in an expression of fathomless disgust. Peltz and Rathbone, familiar faces on US TV, speak their lines with awkward emphasis, as though halfway through their first acting lesson. Ringer, playing the Avatar, has the charm and gravitas of Stewie, the baby criminal mastermind in Family Guy. In the frequent fight scenes, he rouses up air and water by making passes in the air with his hands – but it's hard to represent air on film (duh!) and we're left with the spectacle of a sulky bald kid striking taekwondo attitudes in a castle.

More damning is the apparently casual racism of the casting, which caused outrage among US filmgoers. Shyamalan has changed the Asian/ Mongolian/Eskimo main characters into slow-talking white Yanks – but for some reason he's cast mostly Asians to play the horrible Firebender characters. Shyamalan was born in Pondicherry, India, and raised in Philadelphia. What was he thinking?

But then, for all its strenuous exotic travels to foreign lands, the film is idiotically American. When we meet the Northern Water Tribe, their princess is a Californian blond bimbo, straight out of Beverly Hills 90210. When she propositions the smitten Sokka with the words, "Shall we go see what the ocean is doing?" she could be in Malibu rather than in, you know, mortal danger. The atmosphere of mystical transcendence for which the film-makers are clearly striving remains wholly beyond the reach of the leaden dialogue. When Aang says, "I need to talk to the dragon spirit. Is there a spiritual place around here, where I can meditate?" you just know he's hoping to be directed to an internet café.

The battles are howlingly inept. Brave but raggedy villagers line up against their aggressors as if they're being choreographed by blind people. Gouts of water and bolts of fire fly around the screen but hit their targets off screen. (Perhaps it's the 3D.) Black-uniformed armies sweatily duke it out with other armies in an attempt to recreate the Helm's Deep battle from The Lord of the Rings, but to little effect. The plot disappears from trace for long periods, or suddenly hinges on some teeny detail – as when the earthly representative of the Moon Spirit turns out to be a koi carp in a pond. And I'm sorry to say that every portentous mention of the word "bend" and its derivatives ("From the first time that we knew you were a bender ...") reduced grown-up critics to sniggering 11-year-olds.

It's the sole moment of humour or entertainment in this breathtakingly clueless, misconceived, stupid, humourless, unexciting, dim, dumb farrago, the worst film I've seen in years. It makes Clash of the Titans look like Götterdämmerung.

Next Week:

Jonathan Romney looks in on Mother, a new thriller from Bong Joon-Ho, cult Korean director of The Host