The opening shot of director Bruce Robinson’s The Rum Diary — his first film in almost two decades — depicts a small plane flying into a beautiful coastal city hugged by deep blue water and cooked in sunshine. It’s a slick brochure-ready picture of San Juan, but it’s not long before we see the chaotic and dishevelled side of the city too: junky bars, cock fights, newsroom chaos, angry locals, squalid houses and bootleg alcohol.

Robinson, best known for his 1987 wastoid hit Withnail and I, adapted the screenplay from Hunter S. Thompson’s “long lost novel”, which was written in the early 60s but not published until 1998. Semi-autobiographical, like most of his work, Thompson wrote The Rum Diary when he was just 22, before he drenched himself in drug culture and emerged, liver, sinuses and lungs shot to pieces, a collection of stunning gonzo journalism to his name, to put a bullet in his head in 2005, aged 67, a refugee from an older, looser time.

In some respects The Rum Diary is Thompson’s most mature work, bereft of the wild hallucinogenic prose that flowed like rivers of beer and mescaline through his most beloved texts, such as the clap of counter cultural thunder that is Fear and Loathing in Las Vegas, which still leaps from the page like frogs from a dynamite pond. The Rum Diary’s American abroad story focuses on a love triangle and the day to day grind of being an alcoholic and a journalist. The tone was tight, dramatic, tense and brave.

Tonally, Robinson’s film is lights year away from its source material. It’s more interested in sustaining a druggy OTW vibe than being loyal to what the book was about, which was largely Thompson’s romance with Chenault (Amber Head) and his difficult friendship with her lover Sanderson (Aaron Eckhart).

Opening with a scene in which Johnny Depp, playing a version of Thompson named Paul Kemp, lingers in a filthy hotel room wearing nothing but lipstick and underwear, his bar fridge drained of stock, explaining himself to a shocked hotel employee, is an early indication that the film wishes to tap into the Thompson mythology (he was a specialist at trashing hotel rooms) and rejig the source so that it fits in line with the hi-jinx for which HST was known and celebrated.

There is a scene in which Kemp and new best buddy Sala (Michael Rispoli) take a strange hallucinogenic drug — in the form of eye drops — that isn’t in the book. And his friendship with Sala, while in the book, takes on a buddy-buddy partnership Thompson fans know well: that of the extravagant ‘doctor’ and his loyal ‘attorney.’ And, most despairingly, Robinson seems only vaguely interested in Chenault, who is crucially important to the story.

You could argue all of this shows a damning disrespect to Thompson’s book, though Depp (a close friend of Thompson’s) and Robinson wouldn’t have set out to diss the Good Doctor. The Rum Diary can be better explained as a celebratory work that opts for the myth over the man, more interested in channelling the mannerisms of who the subject became rather than who he was — or is, in the reality of the picture — and makes its own muddles and errors along the way.

Once fans of the book come to terms with the disconnect, they’ll most likely have fun with the film’s slippery rhythm and massively entertaining performances from Depp and Rispoli. Their blubbery energy commands the film, steers it this way and that, through druggy madness to alcohol-clouded conflict to moments of asinine drama and plot trails that go nowhere. With them in the front seats this strange and flawed film is always an entertaining ride, always watchable, and kind of addictive — even when it disappoints.

The Rum Diary’s Australian theatrical release date: March 15, 2012.