The Lord of the Rings: The Two Towers (12A)

We're all doomed: fear and loathing on the road to Mordor

What are the Ents going to look like? That's the anticipatory cringe for any Tolkien fan faced with a film version of The Two Towers. The corresponding question in The Fellowship of the Ring – how on earth will they tackle wacky old Tom Bombadil? – was dealt with by simple excision, but the Ents are too central to the story to omit. It's perfectly possible to read The Lord of the Rings believing in Treebeard without ever having visualised him (is he an anthropomorphic tree, or a dendromorphic man?). There are a controlled number of humorous situations in this mostly very grim film, but any whiff of whimsy or silliness, like a fart at High Mass, would swiftly dispel the magic.

So it's a relief to report that striding, creaking Treebeard, with his Archimboldo face composed of boles and mosses, is a satisfying creation: comical in his slowness, but also wise, and frightening. That Tolkienian note of holy awe, mixed with rustic humour, is very hard to pull off. Though the action in this second part of the trilogy moves decisively towards the world of men, the many ways of being not-human are fully explored.

Problems of visualisation aside, the fundamental question at this half-way point is whether the director, Peter Jackson, can create an artistic whole out of the middle of the sandwich. Will the magnificent filling, pressed on both sides by the great crusts of "the quest begins" and "the quest is ended", just seem shapeless?

No, the film is triumphant on its own terms. The narrative has an unstoppable forward thrust – it's not just a question, as in the Harry Potter sequel, of returning to a place we know and renewing our acquaintance with the strange folk there. The sundered Fellowship, split into three, pushes like a trident deeper into new and dangerous territory. For the first half of the original book, the reader is left in the dark about the fate of Sam and Frodo. They have disappeared from the text and from our ken, and when they next appear they seem isolated and scared.

Jackson opts for a less daring, more conventionally cinematic way of telling the story, cutting rapidly between all three segments at once. This was perhaps an inevitable choice, and it certainly gives the action a reassuring narrative flow, but the effect is to leach much of the despair and loneliness out of the Hobbits' quest. Tolkien's great skill is to make you think, no matter how many times you re-read his book, that perhaps this time it won't work out after all. The message of the second book is There is no hope (but let's try anyway), and Jackson is faithful to this, but the wham-bam style of epic works against him.

Jackson makes other artistic choices at variance with the book, but they rarely feel like violations. Most of them make you reassess the story, draw your attention to unnoticed subtleties. A few of the characters seem even richer, more profound than Tolkien's own conception. Viggo Mortensen finds an astonishing stillness and poise at the heart of Aragorn (he's a bit of a stiff in the novel). This deep love of peace is what drives him to fight, a paradox which makes him more kinglike than any other character. In brief divergence from the book, he's reunited, in memory or dream, with Arwen (is Liv Tyler's spade-like face actually getting longer with each instalment?). This lovely interlude softens the character, redresses an awkwardness in Tolkien, and breaks up the relentless militarism.

Elijah Wood (Frodo) is visibly losing life force as the quest continues; drooping, swooning, a spooked and spooky figure. Gollum is a wonderful creation: voiced by Andy Serkis, and given the most heartbreakingly expressive face, he's far more than a digital effect: he's really there, taking up space, displacing air (part of the impact comes from the meticulous care with which all the creatures of Middle Earth are scaled relative to one another). Gollum is a vile mixture of servility and malice, yet watching him being beaten, throttled, kicked by almost everyone he encounters is as distressing as watching a child being hit. Frodo, for all his faults, is kind to Gollum, seeing in him his own disturbing likeness; Sam, for all his virtues, is cruel. Gollum has been corrupted by his "precioussss", but Jackson strikes an almost theological note in suggesting that his self-centred inability to forgive is really what's corroding him.

There are several hazards which Jackson avoids. This episode, with its histrionic battle scenes and Viking fetishism, Middle Earth having a simultaneous bad hair day, could have degenerated into a heavy metal video aesthetic. As it is, only Faramir's shaggy perm seriously enters Spinal Tap territory. Another error would have been to make this a kind of postmodern pot-pourri of cinematic in-jokes. When Aragorn – missing, presumed very dead – turns up late for a battle, all Legolas (Orlando Bloom) says is: "You're late." That this hoary old line is delivered in Elvish redeems it. Gimli is reduced to a comic character – there's another discussion of when it's permissible to "toss a dwarf" – but there's otherwise nothing camp or arch to leaven the tense moral seriousness.

The Two Towers is a satisfying and wholly gripping drama in its own right. Inevitably, Janus-like, it looks back to the Shire, and forward to the hour of doom. And after three hours, you will too.