No, this isn’t a sequel to Mr. Magorium’s Wonder Emporium. It’s the latest from Terry Gilliam, whose best work (Time Bandits, Brazil, Twelve Monkeys) seems to be behind him, and it’s the last we’re going to get from Heath Ledger. Not surprisingly, the whole enterprise is suffused with a certain poignancy. Imaginarium is also a visual pageant showcasing CGI worlds far more painterly and original than, say, that of Avatar. But for a film wedded to the theme that storytelling makes the world go ’round, it’s weak in that department.

“You can’t stop stories being told,” Doctor Parnassus (Christopher Plummer) tells Mr. Nick, aka the devil (a perfectly cast Tom Waits), in one early scene. The premise of the script, by Gilliam and his old collaborator Charles McKeown, appears to be that stories redeem us regardless of whether they’re true. That even goes for the various religious narratives of salvation and damnation.

A showman-cum-mystic, Parnassus has devoted his life to the telling of one such fable. He drives around London with a horse-drawn cart and actors, putting on a mobile morality play that wouldn’t have been out of place in the Middle Ages (though the sets and costumes belong to Gilliam’s steampunk Victorian aesthetic).

This being present-day London, however, passersby have no interest in a preachy theatrical. (Presumably they prefer their simplistic morality tales in the form of pro wrestling bouts.) Spectators hurl drunken abuse at Doctor Parnassus, while his nubile daughter (Lily Cole) dreams of leaving the cart for the life depicted in her favorite shelter magazine.

Into the gilded shabbiness of their world comes Ledger’s character, an opportunist who isn’t quite what he seems. To be honest, who and what he is remain somewhat unclear. But the important thing is that, by promising to spiff up Parnassus’ show and make it palatable to modern audiences, he becomes an antagonist of sorts. His name is Tony, and Gilliam has said he represents Tony Blair.

This implied satire never comes to fruition, and not just because Johnny Depp, Jude Law and Colin Farrell stepped in to play scenes shot after Ledger’s death. Those switcheroos are surprisingly smooth, perhaps because all four actors hammed up their shared role in the antic manner associated in England with holiday pantos. (Depp, for one, manages an almost scary impersonation of Ledger.)

The transformations are adequately explained by the script, but that’s more than you can say for most things that happen in the Imaginarium. When the action shifts into a surreal world of fear and fantasy — the show inside the show — Gilliam stages crackerjack setpieces. (How about a chorus line of cops trying to recruit a group of mobsters by high-kicking through a number called “We Love Violence”?) But, as in his hectic epic The Adventures of Baron Munchausen (1988), none of this matters as much as it should. Improvisatory mumbling and chaotic physical comedy take the place of character development and clear-cut conflict.