For all its fantastical elements, Terry Gilliam's latest stream of sub-consciousness will be chiefly sought and remembered for Heath Ledger's swansong. Or will it?

Because this is not the mesmeric Ledger of The Dark Knight. His death from an overdose of prescription drugs during the film's production strongly suggests there was more on his mind than acting. It's a supposition borne out by his uncertain performance, typified by an accent that wavers between Sydney and Sydenham.

Eerily we first encounter his character – Tony - hanging and left for dead under a bridge on the Thames. He is saved by the performers of a passing gypsy-style sideshow, a Baroque curiosity known as The Imaginarium of Dr Parnassus.

They are callow Anton (a vaguely irritating Andrew Garfield), the diminutive Percy (Verne 'Mini-Me' Troyer) and Valentina (the surprisingly un-irritating model Lily Cole), daughter of Dr Parnassus himself (Plummer – think Dumbledore as a down-and-out).

The preoccupied Parnassus plays no part in Tony's rescue. You see, when Valentina turns 16 in three days' time, she will become the property of the diabolical Mr Nick (Waits, great), with whom Parnassus has made several dubious deals dating back centuries.

Successful bets have gained Parnassus immortality and the ability to guide people through their wildest dreams in his Imaginarium. The good find light and wonderment. The bad have a nightmare.

But with Valentina's soul at stake, Mr Nick makes another proposition: the first to gather five souls in the Imaginarium wins. However, since the sneaky devil always bends the rules, it's a bet that Parnassus cannot win.

Which is where Tony comes in. A charity founder who fell from grace, he offers his entrepreneurial expertise to help Parnassus attract unsuspecting punters. But is Tony driven by the need for redemption or, as jealous Anton suspects, Valentina's comely charms?

Anyone who saw the documentary Lost In La Mancha will know that Gilliam's cherished Don Quixote project was, to quote Blackadder, strewn with cowpats from the Devil's own satanic herd.

And following the untimely departure of his leading man, the world's unluckiest director was knee-deep in it again. But rather than writing off the production, he reworked the plot and invited Depp, Law and Farrell to play the Imaginarium incarnations of Tony. After all, he was always intended to be two-faced. Now he's four-faced.

The conceit fits well into a fever dream that flip-flops between the real and the surreal. One minute you're flying through Dali-esque dreamscapes with giant jellyfish, the next you're at the checkout in Homebase. It couldn't be the work of any other filmmaker.

But with the wager between Parnassus and Mr Nick being made so bafflingly unclear, it's erratic to the point of tiresome. Like the cartoons of Monty Python, the results are weirdly inspired and fitfully amusing but mostly unfathomable.

Such is the delirium of Doctor Gilliam.

Elliott Noble