The Imaginarium of Doctor Parnassus (12A)

A strangely underpowered effort from Terry Gilliam

Heath Ledger's final completed performance, as well as his most inspired one, was as the Joker in The Dark Knight. But it is The Imaginarium of Dr Parnassus, which the actor was working on when he died in January 2008, that can boast of featuring his last filmed material. There's not much else it can boast about. Ledger's performance is one of many provisional elements in a picture that feels at best under­powered, at worst unfinished.

The director, Terry Gilliam, reconfigured the script he wrote with Charles McKeown so that Ledger now becomes one of four people playing the same part, along with Johnny Depp, Jude Law and Colin Farrell. I suppose Gilliam was forced to hire whichever jobbing actors happened to be at a loose end that week, but then beggars can't be choosers.

The film begins promisingly, with the disorientating sight of a rickety, horse-drawn wooden caravan pulling up in front of a London nightclub. This is the travelling sideshow of Dr Parnassus (Christopher Plummer), a 1,000-year-old sage who doesn't look a day over 998. Together with his company - his daughter, Valentina (Lily Cole), the showman Anton (Andrew Garfield) and the sniping dwarf Percy (Verne Troyer) - the doctor offers bystanders a tour of their own imaginations.

Through a Cocteau-style magic mirror, they step into a world shaped by their own desires. This is where a Gilliam film should go into orbit, but the fantasies are oddly listless, the ideas behind them mundane. A drunk staggers through a river of empty bottles, and a child weaned on Nintendo opens fire on a candy-coloured landscape. Compare this to the warped spectacle of other imagination-bound pictures such as The Cell or Dreamscape and it can only look insipid. When a fur-coated, middle-aged woman marvels at a metropolis of giant shoes, we could be witnessing a nod to Katherine Helmond's leopard-print heel, worn upside-down as a hat in Gilliam's Brazil, or just the limitations of the director's satirical mind.

Into the company's hermitic life drifts Tony (Ledger), a white-suited amnesiac who offers to improve the show's commercial fortunes. Once Tony himself enters the mirror, we see him variously as a slick gigolo (Depp), an ambitious businessman (Law) and a corrupt philanthropist (Farrell). None of these A-list understudies makes much impression, and no one on screen is as vivid or lively as Andrew Garfield (best known from Channel 4's Boy A). But then Tony is a thankless, shapeless role. Given Gilliam's public spats with studios and financiers, it's tempting to wonder if the character isn't a producer manqué, especially when he advises Dr Parnassus to "meet the public halfway", though this isn't developed.

It's regrettable that Ledger's first scene here shows him hanging from Blackfriars Bridge at the end of a noose, with Dr Parnassus telling his would-be rescuers: "Leave him be, for God's sake, he's dead!" Maybe Gilliam kept that line as a comment on our appetite for dead celebrities, just as he shows a flotilla of gondolas bearing the immortal deceased (Rudolph Valentino, James Dean, Princess Di). Yet the hint doesn't mature into an idea, let alone an argument. But then the movie suffers from a general sense of enervation. The plot hinges on a bargain between Dr Parnassus and Mr Nick (Tom Waits), in which the prize is Valentina's soul, but what should be a Faustian horror story plays more like a low-key "Rumpelstiltskin". The basic requirement of any devil figure is that he should be diabolical, but Gilliam never reveals a terrifying side to Mr Nick. There's no urgency in the jousting between him and Dr Parnassus, no convincing threat to Valentina, and not just because we suspect eternity would be better spent in the company of Waits than Plummer. (Well, what would you choose? "Edelweiss" or Swordfishtrombones?)

It's typical of this ramshackle director that the film's neatest joke should be buried in its small print. Midway through the picture comes a curious set piece, part Monty Python and part Michael Clark, featuring a troupe of nightstick-wielding bobbies in miniskirts and fishnet tights. Not until the end credits roll do we learn that their big musical number, "We Love Violence", was performed by the Sir Ian Blair Memorial Choir.