The imagination of Terry Gilliam is a surrealistic place that tends toward the unruly. "The Imaginarium of Dr. Parnassus" follows a similar path.

Though rambling and at times self-indulgent, its wit and pageantry, boosted by Heath Ledger's final performance, render it irresistible.

Dr. Parnassus (Christopher Plummer) is a quasi-magical figure with a knack for looking into the minds of others and seeing their most fervent dreams.

He pilots a ramshackle caravan on wheels, transporting a nomadic theater troupe made up of his teenage daughter, Valentina (Lily Cole), her would-be suitor and co-star, Anton (Andrew Garfield), and a sardonic announcer, Percy (Verne Troyer). Through his wizardly divination, Parnassus offers all comers the chance to leave their humdrum existence behind for a while and enter the world of their imagination — a landscape either bleak or sumptuous — simply by stepping through a magical mirror.

The conceit is a variation of Lewis Carroll's "Through the Looking Glass," but the film proceeds in a very different direction. Dr. Parnassus has his own dark secret and is beholden to the nefarious Mr. Nick (Tom Waits), who is poised to collect the prize Parnassus gambled on many years before.

One day the acting troupe rescues the mysterious amnesiac Tony (Ledger), whose origins and motives are unclear. Gilliam nimbly juxtaposes modern-day England, complete with drab shopping malls, and a mesmerizing fantasy world.

The story, written and directed by Gilliam ("Brazil"), has an abiding compassion toward artists, which is all the more poignant in the wake of Ledger's death in January 2008 of an accidental overdose of prescription drugs.

After his star died, Gilliam set about retooling the script, and his revamping is seamless. Three actors — Johnny Depp, Colin Farrell and Jude Law — portray various aspects of Tony's character after he steps through the mirror. Each gives a believable and energetic portrayal as Ledger's alter ego.

"Imaginarium" may spin at times into bewildering territory, but it's still an inspired adventure.