Film Review: Django Unchained is a blood-filled revenge caper in need of a director's cut

Roots was never like this. But then did we really expect a slave epic by Quentin Tarantino to be anything other than, well, Tarantino-esque? Django Unchained is a film about exploitation and brutality in the antebellum South, brimful of those signature touches that have made the QT name.

In parts it is absorbing and even compelling, in the manner of a violent pulp thriller. You get the impression, nonetheless, that this is a filmmaker who would rather toy with an audience than take them seriously. Like the eternal joker who spins an outrageous yarn just to see the look of credulity on his listener's face, Tarantino's payoff is always the same: "I'm just kidding!"

And yet it begins very promisingly, much as Tarantino's previous outing Inglourious Basterds did, with a long set-piece mingling civility with menace. The year is 1858, the place a wood somewhere in Texas, through which a band of slaves is being marched, shivering, in chains.

Out of the mist emerges one Dr Schultz, a dapper German dentist turned bounty hunter, who stops to ascertain whether there's a slave among them who knows a certain plantation he's heading for. The two white slavers in charge are impatient with this stranger and his precise, courteous English, delivered with characteristic relish by Christoph Waltz, who bagged himself a Best Supporting Oscar for his urbanely psychopathic Nazi in Basterds and sets the tone for this movie with his magnetic self-assurance and charm.

The scene ends badly for the slavers, but Schultz has found his man, a slave named Django (Jamie Foxx) who agrees to help him hunt down three outlaws in exchange for his freedom. Bounty-hunting, he explains, is "a flesh-for-cash business", just like slavery. Django is also on a quest, for his wife, a German-speaking slave named – to Schultz's surprise – Broomhilda (Kerry Washington). Like him she was a runaway, and branded on the face with a telltale R.

So begins an unlikely partnership of white German and black American, whose appearance, mounted and armed, draws astonished comment from local townsfolk – "ain't never seen a negro on a horse". (Negro, by the way, isn't the word they use). Their rough dispensing of justice makes them enemies, too, notably a Colonel Sanders type slave-owner (played by Don Johnson) who also happened to be the employer of that bounty-hunted trio.

There's a righteous smack to this justice, of course, even if it bears scant relation to history. Tarantino is composing a fantasia on the Black Man's Revenge, just as Inglourious Basterds imagined a squad of roughneck Jewish soldiers stomping on the Nazis. Story is this filmmaker's lodestar, and if he can garnish it with sly nods to other movies and other genres, so much the better.

Django Unchained delights in referencing the Spaghetti Western in particular, not so much in its heat and dust as in the Morricone music, the sudden zooms and close-ups, and the exorbitant spectacle of bloody death. When bodies get drilled by bullets here they don't merely fountain blood, they explode with it. Is it worth complaining about his obsession with violence? Blood is as integral to Tarantino as fog was to Turner; you can't imagine his work without it.

The problem with Tarantino in general, and Django in particular, is one of tone. You never feel sure where it lies. For example, a scene featuring a scary-looking posse of masked Klansmen degenerates into farce when they start bickering about the size of the eye-holes in their bagged masks. It's a typical Tarantinoid riff, like the argument about the Madonna song at the start of Reservoir Dogs. Some find this garrulous back-and-forth funnier than others, and that's fine. But Tarantino hasn't found a way to integrate humour with horror, which becomes quite extreme in the latter half.

Django and Schultz have found their way to the plantation of Calvin Candie, a spoilt Southern aristo played with a devil's twinkle by Leonardo DiCaprio. Candie's thin veneer of affability hides a sadist who allows a man to be torn limb from limb by rabid dogs, and who revels in "Mandingo" wrestling – a fight to the death between slave combatants. The obscene degradation of this is plain, yet its impact is dulled by the atmosphere of moustache-twirling campiness surrounding it.

Interestingly, DiCaprio isn't even the worst of the villains. That would be Stephen, the "house slave" played by Samuel L. Jackson in an unsettling compound of obsequiousness towards his master and viciousness towards his fellow negroes. His outrage at the idea of Django staying "in the big house" is both a comical skit on below-stairs uppityness and a corrective to the cinematic ideal of the black man as martyr/saint. It's a risky characterisation on Tarantino's part – not many others would dare.

The boldness of this subversion is undercut, however, by the quickening descent of the film into cartoonish overkill. Django himself becomes a stylised avenger, less real than the brave but bewildered figure whose fate initially troubled us. The shoot-'em-up option becomes the driving imperative.

Tarantino can do many things, but editing is not his forte. He doesn't know what to leave out, and after a gripping first hour the film loses itself in detours and sidesteps. After a second hour an ending appears in sight, but there's still another forty-five minutes of mayhem to go. The director even finds time to feature himself as an oafish heavy, complete with strangled Australian accent. He plays alongside a little-known actor named John Jarratt, who starred as the killer in the cold-sweat Oz horror Wolf Creek in 2005. It's one of several cameos with which he keeps himself amused.

Tarantino's knowledge of movies is prodigious in its breadth. He would probably be the best team-mate you could ever have on a movie pub-quiz. But that may also be the reason why his films feel so narrow. He has the technical skills of a grown-up, and the sensibility of an adolescent.