Director: Bruce Robinson

Cast: Johnny Depp, Michael Rispoli, Aaron Eckhart, Giovanni Ribisi, Amber Heard, Richard Jenkins

It is difficult in that, while it tells the story laid out by the then 22-year-old Thompson in his novel, it looks to impose a retro lens on its characters in a story written by a man who had not yet become the Raoul Duke of Fear and Loathing in Las Vegas, but was still a young journalist dreaming of becoming another Ernest Hemingway.

It is also difficult because, without the mythology around Thompson as the mad, drug-fuelled figure we all know to draw on, the film becomes conventional, uncontrolled and dramatically dubious.

This perhaps explains why, even though it was completed in 2009, it is only being released now.

Set in the late 1950s, The Rum Diary begins with the arrival of the young, uncertain journalist Paul Kemp (Depp) in Puerto Rico to take up a job at a local newspaper.

He falls into the company of a crew of like-minded boozy buddies, who introduce him to the more exotic offerings of the island's nightlife . Paul is introduced to playboy entrepreneur Sanderson (Eckhart) and his dangerously beautiful fiance e Chenault (Heard). Sanderson is one of a growing number of entrepreneurs determined to convert Puerto Rico into an island retreat for rich Americans.

They need a good PR man and when Sanderson approaches Kemp to do the job, the journalist has to choose between making money and taking a stand.

This may seem to have the potential for an engaging story about corruption and ideals, but it ends up a patchy collection of decent moments that don't hold together .

There are several darkly comic scenes that showcase Thompson's sense of humour.

The problem is how to convey the energy, delirium and insight of the prose to screen without losing sight of the basic demands of storytelling. Robinson's non-assertive direction doesn't provide the answer.

Gonzophiles might find The Rum Diary interesting and journalists may hanker for the good old days, but everyone else will be hard-pressed to find anything to care about.

• 'The Rum Diary' opens at cinemas today