Demetrios Matheou gives his verdict on Heath Ledger’s last movie.

It’s likely that The Imaginarium Of Dr Parnassus will always be remembered as Heath Ledger’s last film, which he was still making when he died in 2008. Were it not for that tragic aspect, it would be quickly forgotten as one of Terry Gilliam’s more haphazard affairs, a fantasy in which the former Python’s trademark bursts of inspired and lunatic imagination are outweighed by the amateurish.

But the fact that it was completed at all, with a performance by its nominal star which – through careful plotting and the cunning incorporation of a trio of stand-ins – seems almost intact, is to be applauded.

And it’s not often you’ll get to see a character played by a quartet of actors such as Ledger, Johnny Depp, Colin Farrell and Jude Law, right; or, indeed, a song and dance number by policemen wearing fishnets.

It’s a story about imagination, how it can liberate or endanger us. The eponymous Parnassus (Christopher Plummer) is a crumbling old guru with the ability to empower the imagination of others: when someone steps through the mirror of his Imaginarium, it is into a world created by their own fantasies and aspirations, as light or dark as their inclination. Thus they will either be ejected with a new-found feeling of bliss, or swallowed up forever.

Parnassus steers his paying guests towards the former, but the devil himself (Tom Waits), lurking inside the machine, spies a few souls for his collection.

Having fallen on hard times, Parnassus is reduced to leading a ragbag troupe aboard the travelling Imaginarium, including his daughter Valentina (Lily Cole) and dwarfish right-hand man Percy (Verne Troyer), pitching up at fairgrounds and street corners for their clientele.

Being “societally on the margins, and ­narratively with some way to go”, these are not a happy bunch. Yet matters worsen as Valentina’s 16th birthday approaches and – as per the conditions of one of Parnassus’s numerous bets with his old rival – the devil comes to collect her soul. Will the stranger in their midst, the silver-tongued Tony (Ledger), save the day?

It’s a good conceit, which taps into Gilliam’s love of fairytale (he and Ledger first collaborated on The Brothers Grimm) and allows him to express both the vaudevillian strand of his and the Pythons’ work, and his animator’s endless stream of fantastical invention.

Each new entry into the Imaginarium sets him off in a new direction – from a pair of joy-inducing giant stilts that reach the clouds, to a drunken clubber’s nightmare (drowning in a sea of beer cans) and a contest between those dancing police and a Baboushka that is one of the purest Python moments in Gilliam’s solo oeuvre. The faults of the film are in the real world, where the script and direction (particularly the direction of the film’s supporting roles, all of whom are wheeled on with appalling London accents) are so ropey that I thought I was watching a re-run of an old Children’s Film Foundation feature – honourable, but desperately low-rent.

Gilliam’s most successfully realised films are those where his scattergun instincts are reined in by a strong script – Time Bandits, Brazil, The Fisher King, Twelve Monkeys. Here Charles McKeown (who co-wrote Brazil, but also the doomed The Adventures Of Baron Munchausen) doesn’t offer anywhere near enough quality or solidity.

For his part, Ledger, still soaring on the adrenalin high of his Joker, seems to be ad-libbing to his heart’s content. Some of it works, some doesn’t, but he offers again a charismatic presence. Law is the stand-out stand-in, gaily bestriding the world on his stilts, while the veteran Plummer captures the wear and tear of having too much knowledge combined with too little wisdom.