Harry Potter and the Prisoner of Azkaban (PG)

Harry's darker side

The best news about the third Harry Potter movie was that directorial duties had passed from Chris Columbus to Alfonso Cuaron, who made the Mexican road movie Y Tu Mamá También, a stirringly erotic tale of two guys and a girl in search of a mythical beach. Not having read the J K Rowling novel I wondered if this might augur the first tremors of adolescent longing in the Harry-Hermione-Ron triangle: HP gets a dash of sauce at last.

The best news about the third Harry Potter movie was that directorial duties had passed from Chris Columbus to Alfonso Cuaron, who made the Mexican road movie Y Tu Mamá También, a stirringly erotic tale of two guys and a girl in search of a mythical beach. Not having read the J K Rowling novel I wondered if this might augur the first tremors of adolescent longing in the Harry-Hermione-Ron triangle: HP gets a dash of sauce at last.

In fact, Harry Potter and The Prisoner of Azkaban turns out to be entirely chaste - implausibly chaste; given what else goes on in Hogwarts you'd think the magic of romance would tickle at least one or two of these teenage wizards. So far we haven't seen so much as a peck on the cheek. Cuaron does, however, import something different into a series whose blandness under Columbus had become crowd-pleasing to the point of suffocation. He and his cinematographer Michael Seresin have darkened the Potter palette to a medley of blue-blacks, gunmetals and sickly greens, and fitted out the Gothic interiors with a disquieting touch of noir: mirrors, shadows and flickering lights are significantly featured.

The shade of Dickens stalks this comic-grotesque world where youth and innocence are ominously menaced, from the Squeers-like bullying of the Dursleys to the funeral-director chill of Professor Snape, his lines delivered with a wickedly Dickensian relish by Alan Rickman. The very names of Rowlings characters - Cornelius Fudge, Argus Filch, Peter Pettigrew - recall the wizardry of Boz. One can even hear his sense of the macabre in the announcement by Dumbledore (Michael Gambon, replacing the late Richard Harris) that a decrepit teacher will be retiring "to spend more time with his remaining limbs".

The plot of this new instalment concerns the escape from prison of an infamous wizard named Sirius Black, who is believed to have colluded with the evil Voldemort in the death of Harry's parents, and is now bent on killing Harry too.

As protection against Black the school is patrolled by the shadowy Dementors, wraithlike presences that suck the soul from their victims - like divorce lawyers, only not so expensively dressed. Kinder help is at hand in the person of Professor Lupin (David Thewlis), a tweedy gent who teaches defence against the dark arts, which sounds a lot more fun than double physics, and will train Harry to ward off the indiscriminate soul-suckers.

The news is less good from Hogwarts' other recent arrival, Sybill Trelawney (Emma Thompson), a daffy myopic with flyaway hair who reads tea-leaves and sees "the Grim" at the bottom of Harry's cup: a harbinger of death. "Divination is a very woolly discipline", sniffs Hermione, ever ready with a magisterial put-down. Emma Watson has grown into her role more convincingly than Daniel Radcliffe and Rupert Grint have grown into theirs, seeming no longer the proper little madam but a smart cookie with hidden resources; she's braver than Harry and Ron, too, delivering a splendid uppercut to the jaw of loathsome Draco Malfoy (Tom Felton). It still baffles me why she isn't the star of the show, but I suppose it's a bit late to begin rebranding the franchise.

The allure that production designer Stuart Craig creates around Hogwarts and its environs is thrillingly sinister, and certain elements of its funhouse gaudiness are ingenious, such as old portraits inside, which the sitters fuss and complain, or, in the case of the Fat Lady (Dawn French), are abducted from one picture to another. Inanimate objects have a life of their own.

A criminal's face on a "Wanted" poster cackles in silent scorn, and an ancient map is suddenly dotted with freshly inked footsteps. Set against this is the frantic Hogwarts' house sport, Quidditch, rendered even less comprehensible than before by dint of being set during torrential rain; caped figures wheel and swoop on broomsticks, but the action isn't sufficiently well staged to know who's doing what, or why.

Indeed, the film is at its strongest when it appears to be doing nothing at all, such as the quiet scenes in which Harry talks to Lupin about his lost parents; no fancy special effects here, just the drama of two people in conversation.

It is also one of those rare films in which the appearance of a central character is delayed until the tension becomes insupportable - think of Orson Welles in The Third Man, and Marlon Brando in Apocalypse Now, performances that were paradoxically enhanced by their reduced screentime. We have been reminded of Sirius Black's malign reputation from almost the first frame, yet it's a full hour and forty minutes before the man himself shows up. Gary Oldman plays him as haggard and malnourished as Raskolnikov; we don't hear how he occupied himself for 12 years as a prisoner of Azkaban, but it certainly wasn't personal grooming.

Impressive as his belated entrance is, Oldman isn't really allowed to grasp the movie as Welles and Brando did before him. Indeed, his thunder is partially stolen by a second, unheralded appearance, this one by Timothy Spall, who has allegedly spent most of the movie disguised as Ron's pet rat. The final quarter, with its confused scenes of transformation and escape, is something of a disappointment; not only does the mystery of Sirius Black feel slightly underwhelming, it repeats a whole section of the plot as Harry and Hermione retrace their steps through the events of the previous 20 minutes.

So just when you expect the movie to speed up to a climax, it actually slows down. Nevertheless, I have a feeling thiswon't bother the Potter devotees and perhaps even agnostics will find a certain mild enchantment in this latest.

If it was too much to expect erotic heat, Cuaron has imbued a touch of the night into Harry and rendered the series a good deal more intriguing.

'Harry Potter and the Prisoner of Azkaban' is released on Monday