Near-perfect Potter

This film, like the novel, is by far the longest and scariest episode so far. Kiddies who coped with the first two may find this one too horrific to bear. The Dementors are enough to give anyone nightmares, and I mean that as a compliment.

Hogwarts, exquisitely designed by Stuart Craig, looks bigger and more beautiful than ever, but it also has a more Gothic, menacing feel.

In this episode, Harry Potter is entering stroppy adolescence, and his horrible Uncle Vernon (Richard Griffiths) and snobbish Aunt Petunia (Fiona Shaw) are rightly more scared of him as an angry young man than they were of him as a child.

Harry plays a magical practical joke on the thoroughly deserving Aunt Marge (Pam Ferris in full battleaxe mode), turning her into a human barrage balloon.

Fearing punishment, Harry runs off into the night and is rescued from a black dog by a fantastical triple- decker bus, which takes him to the Minister of Magic, Cornelius Fudge (Robert Hardy). To his surprise, Harry isn't punished for illegally using magic on a defenceless muggle, but is packed off to a third year at Hogwarts School of Witchcraft and Wizardry.

Thanks to a brief meeting with his friend Ron's parents the Weasleys (Julie Walters and Mark Williams), Harry realises that the reason he is being given preferential treatment is that madman Sirius Black (Gary Oldman) has escaped from prison and is trying to murder him.

Hogwarts is defended from Sirius by the Dementors who seem to have an unhealthy interest in - and effect upon - Harry. Fortunately, Hogwarts' new Defence Against the Dark Arts teacher Professor Lupin (David Thewlis) is on hand to shield him from their worst effects.

Not only Harry has problems at Hogwarts. Ron (Rupert Grint) keeps losing his pet rat Scabbers.

Harry's arch-enemy Draco Malfoy (Tom Felton, much improved) is determined to get Hagrid ( Robbie Coltrane) fired and his hippogriff killed. And Hermione (Emma Watson) has developed a mysterious knack of seeming to be in two places at the same time.

The rest of the plot has ingenious twists and turns. It introduces Emma Thompson in full Margaret Rutherford mode as the scatty Divination Professor Trelawney and the great Timothy Spall in a role whose nature I shall not reveal.

Most of the favourites are back, including Alan Rickman as the ever-sinister Snape, Maggie Smith as the stern Professor McGonagall, and Michael Gambon taking over from the late Richard Harris as Professor Dumbledore.

I've always had reservations about the child leads and not all their line-readings are perfect even this time round, but they're all learning on the job.

Rupert Grint has conquered his tendency to pull faces and is now able to pitch lines at a level that doesn't suggest he's desperate to raise a laugh. As a result, he's quite a bit funnier.

Daniel Radcliffe copes reasonably well, and certainly very likeably, with Harry's growth into a confused adolescent and a much more formidable wizard. He doesn't always appear to have a firm grasp on the through-line of his role, but he's helped unselfishly by the grown-up actors, especially Gambon, Thewlis and Oldman, all of them excellent here.

And Emma Watson has real star quality as Hermione. She's improved the most of the trio at giving the impression of being, not acting, and gets the biggest laughs with her awkward transition from anxious nerd to action-woman.

The first two Potter films were definitely children's movies, for good and ill; this one has a darker tone, and more of a mythic quality.

Parents of small children should think twice before exposing them to scenes that 30 years ago would have incurred an X certificate, but creatively this more mature approach is all to the good.

The first two films were highly successful exercises in commercial calculation; this one, though no less worthy of being a box office blockbuster, is starting to look uncommonly like art.

The film officially opens nationwide on June 4 but previews can be seen in many cinemas from May 31