Harry Potter and the Deathly Hallows: Part 1 (12A)

Starring: Daniel Radcliffe, Emma Watson, Rupert Grint, Ralph Fiennes

It feels like the longest, deathliest panto ever made – and still it isn't over. That Warner Bros decided to eke out the franchise by dividing J K Rowling's final book Harry Potter and the Deathly Hallows into two parts is not surprising. What studio would willingly smash this golden egg, with its ready-made audience and its revenues?

Perhaps I should be grateful they didn't split it into quarters, with an instalment for each season of the year. Multiplicity abounds, in any case. In an early scene, the Potter allies assemble in a living room and turn themselves, via a potion, into a platoon of Harry doubles so as to decoy the airborne Death Eaters. Seven Harry Potters in one room: it's like our bespectacled hero has become some Middle Eastern tyrant terrified of assassination.

What's become more apparent as the saga has worn on (this is the seventh in the sequence) is that the film-makers are not prepared to make any concessions to the uninitiated. In short, if you haven't read the books, you'll have virtually no idea what's going on. If you must see it, my advice would be to accompany a likely eight-year-old who can supply the lowdown on the terminology, the metamorphoses and the convoluted story as it relates to its six predecessors. For myself, a sort of cartoon cloud in the shape of a question mark floated above my head for two and a half hours.

I wasn't joking about the panto, either. It would probably be quicker to list the British character actors who don't appear than the ones who do. Some are blink-and-you-miss-them cameos: Fiona Shaw, John Hurt, Helen McCrory, Richard Griffiths. Some characters get more screentime, though not enough to establish anything that deserves the name of "character": Bill Nighy (doing Welsh?), Rhys Ifans (doing Irish?), Helena Bonham Carter, Jason Isaacs, Imelda Staunton, David Thewlis, Peter Mullan, Nick Moran, Timothy Spall, Mark Williams and more pop up like ducks in a shooting gallery. I gather Toby Jones was also among them, disguised as an elf called Dobby. (Think Gollum with a longer nose and a pleasant personality.)

Plot-wise, it's still about Harry trying to elude the dread clutches of Voldemort. Given the latter's evil powers, he's certainly taken his time destroying his arch-enemy. You'd almost believe someone has told him to drag it out a bit. Played by Ralph Fiennes in reptilian mode – does he have any other? – Voldemort appears to have cut off his nose to spite his face, literally, though it's his "Horcruxes" that generate interest here. His what? These are alleged to be the secret to Voldemort's immortality, items within which he has hidden "pieces of his very soul", and rather inconveniently can be located anywhere or in anything. So Harry, no longer under the protective aegis of Hogwarts, must search out these Horcruxes before the forces of darkness overwhelm him.

He does this in the company of his doughty companions, Ron (Rupert Grint) and Hermione (Emma Watson), who wear such solemn expressions you'd think they were in a Bergman movie. The trio take the signpost marked "adventure" and end up in a variety of scrapes that always seem to end just before they become interesting. When they are attacked at a wedding reception, they are magicked away by the wave of a wand to our own Shaftesbury Avenue. Under threat at a remote hideout, they again break the space-time continuum by landing in the Forest of Dean. This ability to flip scenes at will not only sabotages the tension, it makes us wonder about its logic. When a crew of "snatchers" pursues them through a forest, why doesn't whoever has got the wand just spirit them away? And isn't it a cop-out to have Hermione suddenly "remember" stray bits of wizard magic when they're in a tight corner?

Perhaps this flouting of consistency is nothing of the sort to fans steeped in the arcana of Rowling's fictional world. What's not so easily explained away is the odd stiltedness of the relations between Harry and his two best pals. This has seemed to me an encumbrance on the series from the start, and it's exposed more than ever here by the outbreak of "feelings" inside the triangle. Grint has not looked a fair match for Watson, but he does his gingery best. When Ron drops out of the story for a while, leaving Radcliffe to take up the role of romantic protector, it's altogether less convincing. Even at this stage, Radcliffe looks an uncertain screen presence; emotional nuance doesn't trouble his face, and his voice remains a very blunt instrument.

The comparison one tends to reach for when Potter comes up is the Lord of the Rings trilogy. That too was ambitiously scaled, lavishly mounted and wildly overcrowded, yet you didn't need to have read a word of Tolkien to be gripped by it. Potter is a closed system, for aficionados only. Admittedly, there are about 100 million of them, and nothing in the world – least of all a bad review – is going to stop them packing out cinemas from here till Christmas, and beyond. But it's impossible to view this film as a seasonal treat: it is too gloom-ridden, too gothic, too goddamned humourless to rate anywhere on a scale marked "fun". True believers will howl at the thought, but this Potterthon is a bit of an ordeal.