David Gordon Green, who directed Your Highness from a script co-written by Ben Best (The Foot Fist Way) and star Danny McBride, describes his new high-concept comedy as the story of a medieval prince who smokes weed and fights dragons.

Right he is. Whether a tale so slight, framed within an adventure of such modest visual sophistication, will leave a lasting impression – especially in the perpetually impaired memories of the movie’s stoner demographic – seems unlikely. But Highness lives and dies not by the sword, but by the charisma of the man so clumsily wielding it, the exquisitely coiffed McBride.

The Eastbound & Down star plays Thadeous, a lazy prince bedeviled by his nobler brother Fabious (James Franco), and yearning to win the respect of his father’s kingdom without putting in any real effort. Rather than prove his mettle by saving a damsel in distress, he’d rather puff his beloved pipe and bask in lascivious daydreams.

Fabious might have been portrayed as arrogant and condescending, a swashbuckling hero swollen with pride, but refreshingly he is not. Neither Franco nor Natalie Portman, on hand as Thadeous’ unlikely love interest, will earn another Oscar nomination for their work here, but both are agreeable presences in a movie that needs its stars to carry some weight.

Franco, flashing a winning smile early and often, is a fine foil to the sulky McBride, and, as in Green’s equally lightweight Pineapple Express, their chemistry yields dividends. Fabious rarely belittles his selfish brother, preferring instead to nurture  the greatness he perceives lying dormant.

Whether the portly Thadeous can live up to these lofty expectations is debatable – his successes, which are few, seem either accidental or lucky, and sometimes both. But Thadeous, unlike Kenny Powers in HBO’s Down, is not beyond redemption. He is obnoxiously self-pitying, but his intentions are decent beneath the buffoonery.

Not that Highness pretends to be a character study. Silly and liberally profane, it never quite rises to the level of satire, but it is an agreeably cheesy, sporadically amusing fantasy that carries with it a lingering whiff of underachievement.

From a cast and crew so talented, one might reasonably have expected some truly subversive fun, a touch of the same demented inspiration that has made Down one of TV’s guiltiest pleasures. Highness is simply too innocuous to reach such heights, and young Thadeous, for all his mildly laughable shortcomings, is no Kenny Powers.