Depp is Thompson's alter-ego Raoul Duke, who heads for the gambling mecca with lawyer Dr Gonzo (Del Toro) and an awesome quantity of illegal substances in tow. The trip's purpose as research for an article soon becomes buried underneath a trip of a very different kind.
The two leads play out of character (Depp eschews his regular heart-throb image to play the balding Thompson, while those who saw The Usual Suspects will be amazed by Del Toro's transformation). Their characters, though well played, are profoundly unsympathetic and boring.
Where the film really scores is in Gilliam's warped re-creation of their drug-induced hallucinations: carpets coming to life, dodgy business with serpents and all manner of sleazy and unpleasant goings on. Destined to become a cult favourite, but mass appeal is unlikely.