The Imaginarium of Dr Parnassus, Terry Gilliam, 123 Mins, (12A)

Terry Gilliam's new film, The Imaginarium of Dr Parnassus, may not be one of his best, but it's certainly one of his most Gilliamesque. Overflowing with puppet theatres and conjuring tricks, ball-gowned beauties and very short men, fabulous myth and sordid reality, it's like a bulging scrapbook of the director's pre-occupations: it's very Terry, even if it's not very good.

The heroes, for a start, could be the descendants of the travelling players in The Adventures of Baron Munchausen. Led by the decrepit and frequently drunk Dr Parnassus (Christopher Plummer), they take their horse-drawn medicine show around contemporary London, unfolding their stage outside Southwark Cathedral one night, and a Homebase superstore the next. Parnassus's doll-like daughter, Valentina (Lily Cole), her besotted friend Anton (Andrew Garfield), and Parnassus's sardonic, metre-high right-hand man, Percy (Verne Troyer) have the job of drawing the crowds. Parnassus then ushers customers into the Imaginarium – a dreamland where all their fantasies are brought to extravagant life.

This is the director's opportunity to fill the screen with skyscraping ladders, hot air balloons in the shape of human heads, and other typically spectacular flights of Gilliamesque fancy, so it's perverse of him to leave such a device in the wings for half an hour while he wheels on a sub-plot about a bowler-hatted Devil called Mr Nick (Tom Waits). Dr Parnassus lost a bet with him, which means that when Valentina turns 16 in three days' time, Mr Nick can claim her. And with all that going on, Gilliam adds yet another plot to the clutter: the troupe rescues a white-suited amnesiac named Tony (Heath Ledger) who's hanging from Blackfriars Bridge.

In a more conventional narrative, Tony would be the audience's representative – our way into Dr Parnassus's phantasmagorical world – but instead he just piles on another layer of confusion to a story that's already got several unexplored concepts and fuzzy characters too many.

The problem is that Gilliam and his co-writer, Charles McKeown, can't decide whether to concentrate on the magic of entering your own imagination, or the danger of gambling with Mr Nick, or the mystery of a stranger who might be a caricature of Tony Blair, so the film putters in different directions without ever picking up speed. And if a plot is supposed to pivot on a wager with the Devil, it's not very sensible to have a Devil who gives his opponents another chance every time they lose.

I suspect that one reason why so many scenes drag on so listlessly is that Gilliam felt he had to use every available frame with Ledger in it, including some improvised chatter that should have been left on the cutting room floor. Ledger died halfway through filming, before he'd shot any of his scenes in the Imaginarium, so you can forgive Gilliam for wanting to keep him on-screen as long as possible. You also have to applaud the ingenuity and perseverance it took to get the production going again after the death of the star.

Gilliam slotted in the idea that people can change their appearance whenever they step into the Imaginarium, so on Tony's three trips through the looking glass, he's played by Johnny Depp, Jude Law and Colin Farrell successively. It's a masterly solution, although when a film's characters are already too indistinct to care about, dividing one of them into four separate men doesn't help matters. And without disrespect to the late Ledger, his half-mockney, half-Aussie mumble just makes you wish that the far more charismatic Depp had played the entire role himself.

On the subject of "what might have been", The Imaginarium of Dr Parnassus left me with a rueful pang that Gilliam wasn't hired to direct the first Harry Potter film, a job he fought his hardest to get. With someone else's iron-clad plotting to channel his cascading invention, it could have been wonderful. But that's another film which, like so many of Gilliam's projects over the years, is always going to be left in our imaginations.

Also Showing: 18/10/2009

Triangle (98 mins, 15)

Melissa George stars as a single mother who’s out on a yacht with friends when events take a sinister turn. Beyond that, the less you know about Triangle, the more you’ll enjoy it. Suffice it to say it’s a crafty example of the sort of chronology bending puzzler which should be followed by an animated pub discussion on how it all fits together.

Couples Retreat (107 mins, 15)

As a rabid Swingers fan, I was chuffed to hear that Vince Vaughn and Jon Favreau were doing another comedy, but Couples Retreat turned out to be a painful experience. It’s a joke-free, plot-free shambles about four couples who holiday together at a tropical resort. Jason Bateman, Kristin Davis and others top up their tans.

Ong Bak: The Beginning (98 mins, 18)

For that matter, I’m a rabid fan of Ong Bak, too, so this in-name only prequel was just as disappointing. Tony Jaa, the indestructible star of both films, also directs this time around, and he’s taken the franchise a long, long way from the original’s knockabout urban mayhem. Ong Bak: The Beginning is a self-mythologising epic set 600 years ago in the jungles of Thailand. The action takes third place to the period costumes and slow-motion shots of rain falling.