Final Heath Ledger film premieres I Full Cannes Film Festival coverage

Terry Gilliam's magical sprawling tale about the transforming powers of the imagination is touchingly dedicated to the memory of Heath Ledger. The young Australian actor tragically died last year before filming was completed, forcing Gilliam to shoot the remainder of his scenes with three other actors - Johnny Depp, Colin Farrell, and Jude Law - playing Ledger's part.

In the scenes he did complete Ledger is a marvel to watch, though his entrance - hanging from a London bridge with a rope around his neck - is bitterly ironic. His character is rescued by an itinerant theatre troupe who live like tramps in a horse-drawn caravan that unfurls into a gaudy stage. Their show is more of a circus act than a play. Dressed in freakish costumes, Lily Cole, Andrew Garfield, and Ledger drag assorted drunks and bemused shoppers from the street and push them through a large fake mirror on their makeshift stage. On the other side of this mirror lies the fabulous imagination of Doctor Parnassus (Christopher Plummer), an ancient sort of Prospero character whose hallucinatory visions have the power to change people's lives forever.

In this sense the film is a romantic homage to the art of make believe, and it grants Gilliam a license to run riot with his mind-bending illusions. But the story that holds these visions together is slim, incomprehensible, and desperately unconvincing. Parnassus has cut a deal with the Devil - a suave, dapper, cigarette-smoking Tom Waits - and now it's time to pay up. In exchange for eternal life, Parnassus has promised to bequeath his beautiful daughter, Valentina (Cole), to the Devil on her sixteenth birthday. He has three days to save her. Meanwhile his two young helpers, Anton (Garfield), and new recruit, Tony (Ledger, who conveniently has no memory of why he was dangling from a bridge), fall in love with the scrumptious Valentina on their dream-like adventures through the looking-glass. The whys and wherefores of their confusing journies into the Doctor's unstable brain are too contrived for words, but most involve Ledger's mysterious Tony trying to escape from Russian loan sharks.

Frankly, the film could have benefited with a lot more hard story and a lot less whimsy. But it is a visually stunning watch. The maverick Gilliam is blessed with extraordinary creative powers. The sumptuous landscapes he paints when the heroes step through the mirror are just breathtaking: a Chinese monastery carved into a mountainside; a pastel-coloured landscapes with ladders reaching up to the clouds; and a world littered with giant castles and square miles of bottles.

The problem is that it is impossible to unscramble what it all means. To smooth over the joins between his four lead actors' performances, Ledger's handsome but inscrutable Tony is physically transformed every time he wanders through the mirror. Yet none of the famous actors, including Ledger, is in the film long enough to enlighten us about Tony's character, or care about his fate. The revelation is Lily Cole, who is mesmerising as the teenage siren, Valentina. It's her tangos with the various Tonys that keep us focused on the romance. Ultimately, The Imaginarium of Doctor Parnassus is a film with a huge heart and a dazzling eye, but it does little for the very thing it is trying to celebrate: the imagination.

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