Sherlock Holmes: A Game of Shadows (12A)

Starring: Robert Downey Jr, Jude Law, Jared Harris

Arguments about Guy Ritchie's trivialisation of Sherlock Holmes two years ago were rendered more or less irrelevant when the film – surprisingly – made over $500m at the box office. Audiences apparently couldn't get enough of Robert Downey Jr's steampunk version of the Baker Street detective, no matter the preposterous anachronisms and nonsensical plotting the film felt at liberty to indulge. It also made this sequel an inevitability.

How quickly it has established its own mood and tempo. Ritchie, perhaps feeling vindicated by his 2009 triumph, has swapped the five writers on the first movie for husband-and-wife team Michele and Kieran Mulroney, but aside from that it's the same again. I'm not sure Conan Doyle would recognise his creation as revived in A Game of Shadows, though it's not his ghost the film sets out to please. This is product designed for a cinema public raised on video games, slo-mo violence and the genial narcissism of the modern star. It is karaoke Sherlock, barely related to the original, yet not without a sense of fun.

It also carries over from the first film a strong erotic chemistry between Downey Jr's dissolute chameleon Holmes and Jude Law's stable, upright Dr. Watson. This is by no means an original tweak – Billy Wilder put a naughty spin on their camaraderie in his 1970 The Private Life of Sherlock Holmes, with Robert Stephens and Colin Blakely – and in Ritchie's case it is quite consistent with his other work, as male-dominated as wrestling or heavy rock. He doesn't seem to "get" women as characters, and here dispatches two in quick succession: Rachel McAdams, reprising her role as Irene Adler, makes a permanent exit on being poisoned, while Kelly Reilly as Watson's new bride is flung from a train en route to her honeymoon in Brighton. That she lands unharmed in the river is small recompense, given that the man who did the flinging was Holmes himself – leaving him alone with Watson in the bridal carriage. Together at last!

The reason why he takes this drastic course of action is soon explained – the train is crawling with hired killers out for Watson's blood – though Holmes clearly relished his swift ejection of Mrs. Watson to "safety". He persists in the belief that his sleuthing partner prefers his company to that of his missus, and he may be right. Meanwhile, those killers are in the employ of the detective's arch-enemy, Professor Moriarty (Jared Harris), who's been organising a campaign of terror around Europe so as to foment something called a "world war". The year is 1891, with Moriarty plainly a man ahead of his time. As is much else here, including a private motor car and automatic weaponry of improbable efficiency: nothing seems to excite Ritchie more than the feral clatter of machine-guns, extravagantly turned on Holmes and Watson (still in the bridal compartment) when a single gunman could have done the job with less fuss.

The action set-pieces, tending to overkill, are no more satisfying than the whippy montages of deduction meant to evoke the brilliant mind of Holmes, racing to its own speedfreak beat. But there is no plausible sense of mental ingenuity here, just as there is no real clue-hunting. What those montages actually convey is Ritchie and his editor whooping it up in post-production. A Game of Shadows only becomes enjoyable when it pauses the plot and gives the stage to its performers, or rather, performer: Downey Jr sports with the role like a jazz musician noodling in and out of a tune. Half the time his Holmes seems bent on keeping himself amused as much as anyone, tippling on embalming fluid or chowing down hedgehog goulash at a gipsy encampment. And he's very funny when called on to join a posse, revealing a lifelong aversion to horses ("dangerous at both ends and crafty in the middle"). Downey Jr is one of the great crowdpleasers, toying with expectations and always delighted to dress up. Here he disguises himself, variously, as a Chinaman, a bellhop, a lady, a lecture-hall snoozer and – this got the biggest laugh of the night – a chair. Yes, a chair.

The star's madcap persona carries a movie that's a little thin everywhere else. Law supplies a tweedy tolerance as Watson, Harris has intelligence but not quite enough malice as Moriarty, Stephen Fry as Holmes's brother looks a lot like a Wilde caricature of a part he once played (did anyone anticipate his nude scene?) Noomi Rapace, star of The Girl with the Dragon Tattoo, attaches herself to the plot halfway, though I've no idea what she was meant to be doing, and from the look of it, nor has she. Otherwise, honours are divided between Philippe Rousselot's wonderful lighting and photography, and Sarah Greenwood's production design, at least in the London sequences: the tatty street markets and fancy interiors put us right in the middle of what we imagine the fin-de-siècle may have looked like.

Even with its enactment of the famous encounter at the Reichenbach Falls, the picture is at least half an hour too long, and nothing about the film or its predecessor convinces me it will supplant Jeremy Brett in the 1980s-90s TV serial as the greatest Holmes of the lot. Downey Jr's is a cheering incarnation, though the only way he could surprise us now would be to play the part in a cape and deerstalker.