You’ve got to admire Terry Gilliam even when his madcap experiments shatter the test tubes. The former Python is the ultimate independent filmmaker. He has worked within the studio system before, often frustrating the moneymen, but you get the feeling he’d rather burn the negatives than conform to their whims. He is not, as they say, a company man.

His 12th feature, The Imaginarium of Doctor Parnassus, will be remembered as Heath Ledger’s last film – he brings manic intensity to his role as Tony, an amnesiac on the lam – and also for the performances of the men who replaced him after his death: Johnny Depp, Jude Law and Colin Farrell. Depp, who shot his scenes in a single day, is particularly effective here, bringing an appealing sense of mischief to the fantastical world of Parnassus.

Describing Gilliam’s adventures in esoterica is never easy, but let me try. Dr. Parnassus (Christopher Plummer) is an immortal showman, navigating the garbage-strewn streets of modern-day London in his Imaginarium, a mobile stage of ornate but precarious design. Joining him for the ride, and participating in his vaudevillian sideshows, are his cherubic daughter (Lily Cole), her would-be lover (Andrew Garfield) and a stern dwarf (Verne Troyer, Mini-Me of Austin Powers fame) who keeps the doctor in line.

Parnassus is deeply troubled. He has made a deal with the devil (Tom Waits) that may cost his daughter her soul. Can the mysterious Tony, discovered hanging by a noose from Tower Bridge, help save her? The doctor believes he can, and casts his lot with a man whose motives are as murky as his past.

Tony pays immediate dividends, luring unsuspecting strangers off the street and through the Imaginarium’s magic mirror, a doorway into a vibrant fantasyland where temptation has its price and virtue is rewarded with a contact high. It is there, rather than in London’s decaying wasteland, that Parnassus and Beelzebub settle their bets, and with Tony on his side, the doctor smells an upset in the making.

Credit Gilliam with the capacity for envisioning such a wonderfully warped universe, but deduct a few points for execution. At their best, his fantasies soar, thrusting us headlong into worlds of limitless possibility. But Doctor Parnassus, written with longtime collaborator Charles McKeown, never quite achieves flight.

The actors implore us to lose ourselves in the Imaginarium, to be seduced by the bizarre and varied places Gilliam imagines, but their enticements are insufficient. The director’s wonderland contains too little wonder. Though sparkling with digital pastels, and filled with the director’s usual surfeit of imagery, it falls short of engaging the mind.

And so we are left with the performances – Ledger, reportedly suffering from exhaustion throughout the shoot, reminding us again of his brilliance, and the others, including Plummer as the boozy Parnassus, following his electrifying lead. Together, they deliver on the movie’s promise of magic, but this Imaginarium, despite its abundance of CGI spectacle, is incapable of matching their efforts.