The Imaginarium of Doctor Parnassus, Cannes Film Festival

When Heath Ledger died while shooting Terry Gilliam's latest film, the director's novel solution must have pleased the financiers. After all, the aesthetically pleasing replacement trio of Johnny Depp, Jude Law and Colin Farrell were not going to be hard work for the picture's marketers. But the industry kept a sceptical open mind. Gilliam films don't lack for surreal whimsy; but would this enforced face-switching prove a fantastical step too far?

The film begins with a tease: "London, England", runs the screen credit as a Victorian-era travelling show, the Imaginarium of the title, is wheeled into view. But it is quickly revealed that this will be a modern-set tale, about the mysterious 1,000-year-old Doctor Parnassus (Christopher Plummer); his beautiful daughter Valentina, a few days shy of her 16th birthday (model-turned-actress Lily Cole); and the fairground attraction's other employees Anton (Andrew Garfield) and Percy (Verne Troyer, still best known as Mini-Me from the Austin Powers movies). As the showman doctor slips into a deep trance, visitors pass through a two-way mirror into a world of their wildest, fondest imaginings; an interactive thrill ride.

The production design from Anastasia Masaro, stepping up as a reward for her work on Gilliam's low-budget oddity Tideland, is appropriately lavish. In fact, it is the film's only truly remarkable element. As for the plot (if the two long hours of events and incidents may be deemed worthy of the term) a conflict arrives when amnesiac Tony (Heath Ledger) is found swinging from a London bridge, a rope around his neck. The rescued newcomer brings entrepreneurial flair and fresh ideas to the brand positioning of Doctor Parnassus's wheeled box-of-tricks, earning amorous attention from the cusp-of-womanhood Valentina, and thus the jealous enmity of Anton, who is smitten with her. So far, so Dawson's Creek.

The deeper conflict, it transpires, is between Doctor Parnassus himself and the Devil, aka Mr Nick (Tom Waits), who long ago granted him several favours, including immortality, and now arrives to collect the agreed fee: the lovely Valentina.

We already know from titles such as the flashy, high-concept serial killer thriller The Cell (2000) that jeopardy is less compelling to audiences when the events depicted are only happening inside a character's head. Gilliam opts not to accept that truism, although in fairness there is plenty of ambiguity about the how, where and why of events. And those of us only mildly transfixed by the narrative can at least engage with the film's central question: how on Earth will Gilliam get away with a character that mutates between Ledger, Depp, Law and Farrell? The answer: more smoothly than you'd think. Of the three dreamboat substitutes, Depp makes the strongest impression.

Ledger's performance provides an adequate swansong for loyal fans on pilgrimages to the multiplexes this autumn. But many will exit scratching their heads.