The worlds of Renaissance manuscript scholarship and organised crime come together like a mix of Umberto Eco and George V Higgins in this flawed but fascinating reverie from director and co-writer Julian Schnabel. Switching between monochrome and colour, and freely adapted from the Nick Tosches novel of the same name, it is hilarious and shocking, at least at first, with a quite extraordinary tough-guy role for Gerard Butler. It is a mysterious, scabrous and bizarre adventure in violent larceny and spiritual crisis which unfortunately unwinds in the end into sentimental fantasy. In the Hand of Dante amounts to an epic and self-aware jeu d’ésprit with amazing cameos from Martin Scorsese, Al Pacino and Franco Nero, beckoning its audience over to peep into the fathomless abyss of heaven and hell, to ponder the matters of sin, art and the Mephistophelean bargain involved in the attainment of wealth, power and knowledge.

The film unfolds on two narrative levels. In 14th-century Florence, Oscar Isaac plays no less a figure than Dante Alighieri, the great poet whose Divine Comedy virtually invented the concept of redemption, as he grapples with his artistic and spiritual destiny. And in the US in the era of 9/11, Isaac also plays Tosches, louche author and Dante enthusiast, whose aggressive refusal to compromise has alienated publishers and editors, and who now accepts a freelance job via a kid from the old neighbourhood, where the young Tosches had learned to protect himself by any means necessary. John Malkovich plays a mob boss named Joe Black (presumably like the death figure in the movie Meet Joe Black whom Anthony Hopkins meets); he has come into possession of what seems to be the priceless lost original manuscript of Dante’s The Divine Comedy, discovered by an ancient Catholic priest in Sicily with mafia connections.

Tosches’ job is to journey there, verify its authenticity, spirit the manuscript beyond the reach of Italy’s legitimate national authority and then set up a colossally lucrative private sale. But he must do all this in the company of hideous, loquacious hitman Louie, whose job is to whack any witnesses; he is played with satanic relish by Butler, who drolly doubles in the role of Pope Boniface VIII back in medieval Florence. Butler’s performance in his opening scene is a jaw-dropper: brimming with obscene contempt, he confronts a cringing bar owner who owes him money. And there’s hilarity in the opening scenes when Louie and Nick meet in Joe’s office to discuss the genuine Rembrandt that is hanging there.

The combustible mix of lowlife cynicism and high art provide enough energy and enjoyment to power the first two-thirds of this long film. But in the end it flags, and it’s as if the outrageous black comedy has to be paid for with solemn romantic fantasy. But what a performance from Butler.