Sherlock Holmes (12A), Avatar (12A)

Don't look for the deerstalker. You'd be more likely to spot a hoodie in the new all-action take on Sherlock Holmes, who, in Robert Downey Jr's incarnation, is less cool-headed sleuth than bohemian slob.

If the squalor of his flat in Baker Street is any indication, this fellow could trade as The Minging Detective. For, despite its atmospheric Victorian setting, Guy Ritchie seems intent on giving Conan Doyle's creation a 21st-century makeover – a sort of Bond in wing collars. Stubbled and stocky, Holmes is no longer just a brilliant mind, he's also a bare-knuckle boxer, an athlete, a cracksman and an inventor. The only thing he can't seem to do is personal grooming.

This cavalier reimagining has extended to Dr Watson, once the bumbling sidekick who had to have everything explained to him; now, played by Jude Law, he's a pistol-toting daredevil who likes to brawl and gamble. He also has a fiancee (Kelly Reilly), though she might just be a beard, since there's a noticeable current of attraction between himself and the boss. They're still cohabiting at Baker Street, and it's clear that Holmes is not ready for Watson to leave him. Fans of the Conan Doyle stories will bristle at such liberties, but there's no reason why you can't play the partnership as a comedy. Billy Wilder fashioned just such a thing in his brilliant 1970 film, The Private Life of Sherlock Holmes, which souped up a plot involving dwarfs, German spies and the Loch Ness monster. It didn't make sense, yet it had Robert Stephens and Colin Blakely as Holmes and Watson, a magnificent Miklos Rozsa score and an unexpectedly moving end.

More significantly, Wilder's version of Holmes was funny, which this one most definitely isn't. Ritchie is all at sea because he has no grasp of language as something comic, a problem intensified by the thin overlay of "period". Characters adopt a style of address and manners that no Victorian would recognise, and the actors haven't been drilled to project even the smallest pretence that they're living more than 100 years ago. The plot involves an occult conspiracy and a high-born murderer, Lord Blackwood, played with Dracula-esque relish by Mark Strong, though it's less a narrative than a succession of set-pieces, most of them Bond-like scrapes which our heroes, accompanied by a feisty American lass (Rachel McAdams), contrive to escape without ever quickening the pulse. Ritchie generates energy but no excitement – we never fear for anyone because the danger is not remotely believable. Is there anything good about it? Yes – Sarah Greenwood's production design conjures a seductively grimy London of wharves, workrooms, slaughterhouses and teeming streetlife, but it's a backdrop that goes to waste when what's happening in front is so uninvolving.

I made a deliberate decision not to read any advance reports of Avatar, James Cameron's 3-D epic adventure, which has had a heap of hype even greater than his last movie – and that was Titanic. It's increasingly difficult these days to see a movie "fresh", that is, unadulterated by sneak previews and web rumour. I'd heard only that the movie was somehow "game-changing", and went in hoping that I would be able to cope with the full effect of its mind-blowing otherness.

Two hours and 40 minutes later I emerged to find that my mind, while not underwhelmed, had survived unblown. Visually, it goes right down the shock-and-awe route, striving to fill every inch of the screen. It is, as you'd expect of Cameron, a stringently organised piece of work. The story involves a mission by the US military to a human outpost on Pandora, a moon rich in a mineral, Unobtanium (no, really), that the earth badly needs. The natives of Pandora are the Na'vi, blue-skinned, amber-eyed, pointy-eared, and about 10ft tall; they live off the land, taming the wildlife and worshipping gods in the manner of Native Americans. The military have infiltrated the Na'vi by using avatars, lookalikes who are mind-controlled by humans wired up on a nearby spacecraft, the latest recruit being a paraplegic marine, Jake Sully (Sam Worthington), who, once he's in avatar-mode, can walk, run and hunt with the best of them.

You can take your pick of the allegories. In the despoiling of the Na'vis' land, there's an obvious parallel with what the US did to its native population, and in the lowering presence of jungle-bound marines and floating armoured gunships we see the madness of later American adventures in Vietnam and elsewhere. There are nods to the firestorms of Apocalypse Now, and the gung-ho colonel (Stephen Lang) who's leading the pillage – sorry, mission – is worryingly close kin of Robert Duvall's Colonel Kilgore. To the film's target audience, such resonances will mean less than the fantasy set-pieces in which, say, Sully wrangles a gigantic winged monster – think Buckaroo on a pterodactyl – and his romance with warrior princess Neytiri (Zoe Saldana).

You can't help but be impressed by Avatar as a spectacle, and the combination of motion capture and CGI has been honed to a degree of sophistication that is almost scarily lifelike. Yet, as in Titanic, Cameron's technical wizardry far outstrips his narrative capability, and the last third of the picture goes full-tilt into action mode, blitzing all before it with aerial explosions and hand-to-hand combat. These weren't uninteresting in themselves, but the moral issues – corporate predators versus harmonious tree-dwelling natives, militarism versus humanism – aren't exactly rich in nuance.

'Sherlock Holmes' is released nationwide on 26 December. 'Avatar' is open nationwide