The adjective "dark" has always suggested something sinister, often associated with the Prince of Darkness. But more recently in popular culture, and especially in the movies, it's come to mean deep, serious, mature, dangerous and altogether more truthful, more worthy of intelligent consideration than anything categorised as "light" and thus frivolous and deceptive.

Penumbrously lit by Portuguese-born French cinematographer Eduardo Serra, the latest and penultimate film in the Harry Potter cycle (in fact the first half of JK Rowling's final book) begins with an ominous, Sergio Leone-style close-up of Bill Nighy telling us: "These are dark times." He sounds like any member of the coalition cabinet at the dispatch box, but he is, in fact, Rufus Scrimgeour, minister of magic.

Not long after, he's presenting the orphaned messiah Harry Potter and his two wizardly chums, the upper-middle-class Hermione Granger (Emma Watson) and the lower-middle-class Ron Weasley (Rupert Grint), with mysterious inheritances from Dumbledore, their mentor and former headmaster at Hogwarts School for Wizardry. These gifts will assist them in their imminent apocalyptic encounter with the evil Lord Voldemort that will settle the future of mankind.

Now a decade in the telling, the Potter saga is getting a trifle thin, while its heroic trio are developing bags under their eyes and behave like schoolchildren wondering whether they should spend their gap year chasing dragons or hunting for the Holy Grail. Dumbledore is dead and most of the adults make only token appearances, the chief exception being Helena Bonham Carter as Bellatrix Lestrange.

The forces of evil, with Ralph Fiennes's Voldemort in the chair, gather to decide who'll kill Potter, while the forces of good assemble at Potter's suburban home to plan his rescue and transfer to a safe house. After this, Harry, Hermione and Ron are on their own working out how to find the Arthurian sword (predictably lying at the bottom of a frozen lake) that will enable them to vanquish the Horcruxes, Voldemort's airborne cohorts.

Most of the time, the kids are in the wilderness, a dark, wintry place either on the Pembrokeshire coast or in the Highlands. But from time to time, they transport themselves elsewhere. Most particularly, they enter the normal world of the Muggles, now dominated by agents of Evil and resembling a cross between Lang's Metropolis and Dickens's London. There, they take over the identities of three everyday adults in a manner that closely resembles certain sequences of Christopher Nolan's Inception.

In the absence of the eccentric, outlandish staff of Hogwarts and Voldemort's wicked crew, the film becomes a rather pale affair. Harry, Hermione and Ron, personable as they may be, and the bickering adolescent interplay between them, are not sufficiently interesting to hold our attention. The film's succession of remarkable state-of-the-art special effects resembles a fabulous firework display put on by rather spoilt, ageing children at a dull Guy Fawkes party where the adults have all slipped off inside for a drink.

There are some good scenes. In one of them, entirely dependent on special effects, Alastor "Mad-Eye" Moody (Brendan Gleeson) gives a special potion to Harry, Ron, Hermione and half-a-dozen of their contemporaries that turns them all into clones of Harry to lead the Horcruxes on a merry chase around the country. This is "I am Spartacus" Hogwarts-style. In the one truly magical scene in the film, Harry, Hermione and Ron encounter a new character, Xenophilius Lovegood (Rhys Ifans), a Welsh wizard who explains to them the meaning of the sign of the eponymous deathly hallows and relates the resonant fable behind it.

A quest in the Grimm manner in which three brothers are each granted a wish from Death, his story is accompanied by a breathtakingly beautiful animated sequence combining Indonesian shadow theatre with the silhouette figures used in innovative German animator Lotte Reiniger's fairy tales of the 1930s.

Otherwise, Harry Potter and the Deadly Hallows Part 1 is an inchoate thing that doesn't stand alone and ends abruptly in medias res. It's overlong, padded out and, to one unacquainted with the novel, incoherent. It is also obvious that a number of scenes were planned for 3D, including the opening, in which the camera floats through the Warner Brothers logo, and an impressive shot of a giant snake crawling down Voldemort's conference table to swallow a victim and with her the audience in the cinema.