Lady in the Water (PG) <!-- none onestar twostar threestar fourstar fivestar -->

Dead in the water

Sometimes a movie gets it so wrong you wonder if it might just be a gigantic send-up of itself, and you can imagine the actors on the verge of corpsing, the cameraman's shoulders starting to shake, and the famous, "quiet on the set, please" about to break into howling gales of laughter. In truth, there are no such moments in M Night Shyamalan's latest, Lady in the Water. The cast maintain an impeccable concentration, given the circumstances, and no sudden belly-laugh disturbs their air of earnest, philosophical puzzlement. All the same, the movie is a train-wreck, a calamitous mishmash of fantasy and horror that only a film-maker utterly besotted with his own reputation could have inflicted on the public. It's breathtaking - in all the wrong ways.

You'd have to say this wreck was foretold. Shyamalan's 1998 debut The Sixth Sense came out of nowhere and blew the box-office away, a success that might have turned the head of any 29-year-old writer-director. What followed were ambitious and unevenly composed variations on "the uncanny". He stuck with Bruce Willis, this time as a drab superhero, in Unbreakable (2000); planted Mel Gibson amid the alien corn in Signs (2002); then really pushed his luck with the fairy-tale duplicity of The Village (2004), which mixed Little Red Riding Hood with The Twilight Zone to no great advantage. He kept getting away with it, evidently able to convince those who mattered that his talent brooked no opposition - either he was in sole charge, or there wouldn't be a movie.

That uncompromising stance was finally brought to account when Disney, where he'd made his previous films, read the draft of Lady in the Water and confessed their doubts. The executive in charge, Nina Jacobson, told Shyamalan - quite reasonably - that she didn't get it. In a huff, Shyamalan made a deal with Warner Bros instead, and then there was nothing to stop him revealing that his head had finally disappeared up his arse.

How to begin unpicking its multi-stranded inanity? Paul Giamatti plays the Dickensianly-named Cleveland Heep, the caretaker of an apartment complex near Philadelphia, who one night discovers, in the swimming pool, a mermaid named Story (Bryce Dallas Howard). Technically she is a "narf", a sort of sea-nymph who's been stranded on earth, and must rely on Cleveland's ingenuity to help her return to her native "Blue World". He enlists the aid of various tenants, including a crossword champion, a body-builder, an old Chinese lady, and a kid who can read the runes in cereal packets, all with their own ideas of how to help Story; meanwhile, in the woods beyond the complex, a bizarre demon-dog known as a "scrunt" lurks in wait. The nymph, blessed with visionary powers, knows that another of the tenants, a frustrated writer, will produce a book that bears "the seeds of great change". This genius-in-waiting is played, with characteristic modesty, by Shyamalan himself - no fleeting Hitchcockian cameos for this guy.

As puffed-up as he is, Shyamalan is perhaps not as self-confident as all that, for the nearest the screenplay comes to a villain happens to be, horribile dictu, a film critic. As played by Bob Balaban, he's a prissy, pedantic fellow who thinks he knows better and, in one out-of-nowhere scene, explains to camera the conventions of the thriller, à la Wes Craven's Scream cycle. Unfortunately the comedy falls flat, essentially because it's written by somebody with no sense of humour, though the critic's eventual fate suggests that same somebody has a strong (and pretty juvenile) sense of revenge.

"You have to believe this all makes sense somehow", says one character, as the story tips into an inexcusably tedious spiral of whimsy and feyness - well, you try making sense of the "guild", the interpreter, the eagle and the reams of Chinese folklore. The only thing holding it together is Giamatti's hopelessly sincere performance as the caretaker, keeping a straight face when most would be creasing theirs. Shyamalan might still have clawed back something from the wreckage - it's too woeful a thing to save - had he been inclined to jolt us with a trademark twist, and, for a while, I wondered what might be coming. Could Story turn out to be not a nymph at all but a member of the American aqua-aerobics team who's lost her marbles? Supply your own ending: it couldn't be any feebler than what's on offer here.

A minor controversy has recently irrupted in the film media about the forthcoming trash comedy Snakes on a Plane, which its studio has decided not to preview to the press. Their caution feels misguided, since Snakes is, by most accounts, one of those great "bad" movies that generally promote the gaiety of nations. Even film critics can see the point of them. For future reference, if the studios really want to do everyone a favour, Lady in the Water is just the sort of portentous junk they should withhold from critics; indeed, we would owe them a debt of gratitude. Warner Bros marketing department could simply have slapped a "from the director of The Sixth Sense" on the poster, cast a Da Vinci Code-like veil of secrecy about it and let cinemagoers take their chances. Maybe that's what they should do with the next Shyamalan movie - if they're mad enough to let him make another one.