The preview for Your Highness is a masterpiece of the trailer-maker's art. It does much more than highlight the movie's best gags; it weaves them together, sometimes transposing dialogue between scenes, to create a brilliant, wholly false impression of the film.

All the qualities that the project could or should have had are there on screen for two-and-a half enticing minutes. Among these are the pleasure of seeing an unusual cast that includes James Franco, the comedian Danny McBride, Charles Dance, Justin Theroux, Damian Lewis, and Natalie Portman romp around in medieval costumes, while speaking raunchy half modern, half ye-olde-worlde lines.

Portman in particular has done very little action or action comedy before and the trailer understandably makes the most of her fight scenes and a sequence in which she strips down to a thong and dives into a pool.

Your Highness looks for all the world like a raunchier contemporary update of William Golding's superb The Princess Bride combined with Mel Brooks's Men in Tights and some knowing visual references to sword-and-sorcery films like Conan the Barbarian.

But once respected drama director David Gordon Green, who most recently made the mostly tedious marijuana comedy Pineapple Express (which also featured Franco), and co-writers McBride and Ben Best have made a film with all the weaknesses of something improvised while under the influence of cannabis.

Franco plays Fabious, a handsome, athletic prince, and McBride his lazy, cowardly, but lustful brother, Thadeous - a kind of medieval Flashman.

When Fabious's fiancée Belladona (Zooey Deschanel) is kidnapped by evil magician Lezard (Theroux) they go on a quest to rescue her. On their way to the magician's dark tower they join forces with a mysterious and extremely capable female warrior (Portman) and battle various strange, often naked enemies, including some pale skinned nymphs and a minotaur with an erection.

The script feels like an amateurs rough draft, compared to even the weakest of the Scary Movie and Naked Gun spoofs, and is pathetically dependent on the overuse of the F word. It is emblematic of the movie's tone that the event the main characters go on a quest to prevent - the union of the evil magician with the kidnapped princess - is called "the f**kenning."

The cod English accents employed by Franco (a surprisingly weak comic actor), McBride and Portman are not nearly as funny as they are apparently supposed to be.

Yet, during the long, dull, laughless stretches between frenetic action scenes and bursts of expensive magic effects, you can somehow sense that the filmmakers believed they were pulling the whole thing off, in the manner that people who are high sometimes think that they and everything around them is completely hilarious.

Deschanel, a fine comic actress who specialised in quirky roles (and who has a perfect English accent) is wasted here, as are the Northern Irish landscapes where much of the film was shot.

With the exception of a couple of sight gags, including one involving an absurd amount of blood and guts, everything remotely amusing in Your Highness is in the trailer. Which means that any money you spend on a ticket is wasted, and every minute devoted to watching the rest of the film is time thrown away.