When I found out Danny McBride and Ben Best (co-creators of “Eastbound & Down”) were writing a fantasy satire about a younger brother who’d rather toke it up and have sex with dwarf royalty than be a responsible prince, I was excited. Sadly, a concept is only as good as its execution, and “Your Highness” falls through, only succeeding at executing my sense of humor for 100 minutes that felt so much longer.

Natalie Portman stars as Isabel, the lone bright spot in “Your Highness.” The movie is the first featuring Portman since receiving her Academy Award for her performance in “Black Swan.” (Photo courtesy of Universal Pictures)

Thadeous (McBride) can’t seem to do anything right, always falling short of his brother’s triumphs. But things go wrong at brother Fabious’ (James Franco) wedding when the evil sorcerer, Leezar (Justin Theroux), re-kidnaps the bride, Belladonna (Zooey Deschanel), who he plans to deflower in order to create a dragon (don’t ask, it’s a magical spell thing).

Tired of Thadeous’ selfish demeanor, King Tallious (Charles Dance) forces the lazy sibling to accompany his valiant brother or be banished from the kingdom.

With no combat training, an excessive enjoyment of ladies, and an unnamed herb that relaxes him in a way all too familiar to Mary Jane, they embark on a quest, facing off against nude, forest-dwelling women, a minotaur and the usual suspects that Harry and Frodo do brunch with every other year.

As far as juvenile humor goes, “Your Highness” lives up to its reefer-implying name.

Fadious (James Franco), Belladonna (Zooey Deschanel) and Thadious (Danny McBride) ride off to misadventures in “Your Highness.” The film is directed by David Gordon Green and opened last week. (Photo courtesy of Universal Pictures)

Too bad it’s not smart idiotic comedy (yes, I would argue there is such a thing). The “F” word can be a remarkably effective literary device when used in moderation and in appropriate places, but the dialogue continually reverts back to this word, as if the filmmakers know how flat the script is and feel like this one magical noun/verb/adjective/(occasionally) adverb can make the difference.

Ever since “Freaks and Geeks”, James Franco proved his worth as an on-screen stoner, picking up the mantle made famous by Jack Nicholson in the 1960s. He might’ve overdone it here with more slurred speech than coherent sentences. McBride found his typecast in “Tropic Thunder” and kept it just as strong in “Pineapple Express”, but the overweight, goofy persona only works when he’s got something unique to say.

The one saving grace of the film occurs just before the halfway point when Natalie Portman’s character, Isabel, appears (and it’s about time by that point).

She brings with her many years of enjoyable roles, but this one feels more like a fill in, hurriedly concocted before she begins to show from her current pregnancy.

Sadly, when a film’s high note is the presence of a beautiful co-star, things return to what they were rather quickly.

Director, David Gordon Green, was on to something with “Pineapple Express”, merging pothead comedy with violent cop movie action, but the familiar satiric tone and action gore doesn’t have the character chemistry to keep things as engaging.

Only rare moments tap into the potential hilarity of the fantasy genre, like one such scene when Leezar decapitates a fairy, squeezes out the fairy dust and snorts it like cocaine.

Spoof films go wrong when they become a one-note song with no concern for characters (just ask “Scary Movie” and all of its offspring). “Your Highness” suffers from bland performances provided by great actors.

Performances bounce between dumbed down Shakespearian dialogue and crass references to various sexual acts and drug use. Again, it could’ve been funny if it all didn’t feel so forced.

The problem here is audience. Who is the demographic of choice? If it’s the fantasy lovers, “World of Warcraft” crew or anyone with a love of otherworldly stories, then “Your Highness” has a major identity crisis, as it shoots for fans of crass comedy without providing an effective bridge to this land of awe. It’s pretty clear that these filmmakers aren’t literate in the ways of trolls, goblins, fairies and wizards, compensating with repetitive jokes and replications of cliché fantasy conventions.

Hopefully someone will try this again and will do more than check the popular flicks for ideas, and actually try to tap into what makes fantasy so interesting.

Now, I can’t lie. I did occasionally burst out laughing, but the “ha-ha” to “whatever” ratio is skewed, leaving me more inclined to spend the weekend watching “Bones” reruns instead. It’s really a tragedy, considering how good this one could’ve been.