Like the creaky wagon inhabited by a mystical theatrical troupe that rolls into London early in The Imaginarium of Doctor Parnassus, the film itself wobbles beneath the weight of its own shaggy-dog fairy tale.

Still, there are wonders to behold in director Terry Gilliam’s latest feature, which opens Friday in limited release before expanding in early 2010. Among them, Heath Ledger makes his final film appearance as Tony. While this slippery rogue falls short of Joker-magnitude greatness, the late Australian actor imparts the character with a scruffy charm.

Johnny Depp, Jude Law and Colin Farrell stepped collectively into the Tony role after Ledger died midway through filming. Absent a complete performance by Ledger, Imaginarium’s real star takes center stage halfway through the movie when Gilliam unveils a parade of surreal sequences brimming with the visual bravado common in his early work on Monty Python’s Flying Circus, Brazil and Time Bandits.

The story loosely revolves around an aging, Vaudeville-style impresario/swami named Doctor Parnassus (played by Christopher Plummer) and his daughter Valentina (Lily Cole). From the stage of the Imaginarium wagon, they offer audience members the opportunity to travel through a magic mirror.

Shortly after the pesky Mr. Nick (played by musician Tom Waits, pictured above with Plummer) comes to collect an ancient debt, the ragtag troupe, including cynical dwarf Percy (Verne Troyer) and vaguely talented Anton (Andrew Garfield), takes in a nearly hanged-to-death Tony.

This chunk of scattershot exposition cuts eventually to the chase wherein Tony, Valentina and Parnassus experience a series of larger-than-life nightmares courtesy of that Alice in Wonderland-like mirror.

When Gilliam throws conventional narrative out the window, cranks up the special effects and draws on dream logic for inspiration, that’s when the fun begins.

Gilliam and his team bring a painterly eye to these tableaus. Candy-colored rowboats, inky-black rivers that morph into a king cobra crowned by Waits’ head, brooding mountain monasteries and treacherous crags and hangman’s nooses and stilt races through the clouds. It all flickers across the screen in ways that tap primordial anxieties in the finest dark-fairytale tradition.

As Ledger’s replacements, Depp, Law and Farrell throw themselves into the shape-shifting antihero’s dapper white suit with reasonably strong results and, in terms of sheer ingenuity, Gilliam pulls a rabbit out of the hat simply by completing the movie.

But even as the PG-13 film celebrates the thrills of a rich fantasy life, The Imaginarium of Doctor Parnassus raises an unanswerable question: What would we be seeing if Ledger had lived to give a fully realized performance?

In his absence, venerable 80-year-old Canadian actor Plummer gets in some marvelously morbid moments as the morally conflicted Doctor. And posing as a devilish tormenter, Waits chews the scenery a bit, but that’s OK — he is the devil after all. Model-turned-actress Cole adequately fulfills the story’s feisty princess function, and, like all the characters, Valentina benefits from the densely detailed costuming of Monique

Prudhomme (Juno).

By the time the players have strutted their hours upon Gilliam’s stage, the tale itself seems almost beside the point. What remains is the sound and fury of cinematic postcards dispatched directly from the nerve center of a wondrous imaginatio