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The Imaginarium of Doctor Parnassus” concerns a magical portal that leads to a dreamlike fantasy world, which ends up not the best object for a director whose films are often just such devices. Creating a story around a mysterious and immortal sideshow performer named Parnassus (Christopher Plummer), Terry Gilliam (“The Brothers Grimm”) demonstrates why some filmmakers are best when working with material that contrasts with their interests.

It might surprise audiences that the most enjoyable moments in the film are outside Parnassus' Imaginarium, on the dirty cobblestone streets and alleys of London, where Parnassus' theater troupe winds its way between neighborhoods, trying to attract customers while being followed by a pesky devil named Nick (Tom Waits). Here we find everything best about a Gilliam film: the dry wit, the daring use of fantasy and the sense that the antiquarian and the weird lurk behind any corner. Inside the device, Gilliam cancels himself out, lavishly concocting an infinite realm of the imagination that is frequently unimpressive, stale and tedious.

You could take the 1,000-year-old Parnassus as a representative of Gilliam. His Imaginarium, behind a phony mirror made of a reflective plastic curtain, transports brave visitors into some kind of fourth dimension created by Parnassus' mind, the first of many fuzzy details. The Imaginarium world is — no surprise — just like a movie by Gilliam, creator of “Time Bandits” and “The Adventures of Baron Munchausen.” There is even a scene that recalls an earlier hand-drawn moment from Gilliam's co-directing debut, “Monty Python and the Holy Grail.” But the filmmaker decks his central concept with so many other ideas of varying quality, we lose sight of why it matters, until the movie teeters and creaks like the bulging, rickety horse-drawn cart that carries it.

Into this mix lands Tony (Heath Ledger, in his final performance), a dapper and articulate young man in dire circumstances who has lost his memory. Gilliam introduces him hanging from the end of a noose, a curious decision, though he dedicates the movie to his fallen star. Tony joins the troupe, which consists of former street urchin Anton (Andrew Garfield), a dwarf (Verne Troyer) and Parnassus' lovely daughter (Lily Cole), who develops a fondness for Tony that bothers Anton. Tony, we learn, was once a successful charity organizer, and his newfangled ideas for Parnassus' Imaginarium update its look and rake in the dough.

Parnassus needs the help, because he is in debt to Nick, who met Parnassus when the latter was already an ancient monk living in a deep underground monastery, telling stories that maintained the existence of the universe. There's much more, but it's enough to say that, as a story, “Imaginarium” feels as infinite as the Imaginarium. By the time the angry Russian mobsters arrive you may welcome them as one more bit of enjoyable ridiculousness or be well on your way to throwing up your hands at the entire affair.

“Imaginarium” is a beautifully shot film, highly fanciful and filled with hilarious moments delivered by likeable actors, including a somewhat haunting performance by Ledger, who died during the movie's filming. Gilliam only finished the project with the help of Ledger's actor associates, including Johnny Depp, Jude Law and Colin Farrell, all of whom play different physical incarnations of Tony. The unlucky accident actually enhances the Imaginarium bits, as shape changing is apiece with the disjointed reality and all three stand-ins, especially Depp, bear a resemblance to Ledger. But their presence doesn't alleviate the feeling that there's too much going on for any of it to resonate.

There's no center to this rambling film, a fact most apparent in the wagering element Gilliam adds between Parnassus and Nick, who battle inside the Imaginarium for the souls of visitors. Their choices are supposed to be between good and evil, but it's never clear how, or why. In a typical scene, a wealthy lady walking through a muted Egyptian landscape chooses to take a place in eternity among screen legends riding in Italian gondolas along a Styxlike river, rather than spend an evening with Depp at a “one-nite-stand” motel run by Nick. Nothing about the decision feels particularly righteous or consequential. Maybe she is primarily choosing the wonder of imagination over the mundane pleasure of reality. Audiences curious enough to enter Gilliam's head this time will have to make a similar choice. (PG-13) 122 min. HHIII