"Year One" is a movie that not only seems aimed at 12-year-old boys, but made by them.

Jack Black and Michael Cera star as Zed and Oh, two cavemen who leave their prehistoric village behind and set off into a confused world full of biblical characters, Roman soldiers and plenty of opportunities for gross-out humor. The extremely tenuous plot involves their quest to save two cavewomen, but all of this is really just a backdrop for a zany cast of characters dressed in period garb that - purposefully, we hope - looks like it came directly from a high school drama club's costume closet.

Black and Cera, both of whom have alternated between funny and flat in their previous films, could have been a genius pairing. There is something physically hilarious about the two of them side-by-side: Cera delicate enough to be almost womanly, and Black hulking enough to play a caveman quite convincingly. It's an instantaneous visual depiction of how broad the spectrum of "man" actually is.

But the fun, for the most part, ends there.

As soon as these characters start talking, the script - despite occasional moments of cleverness - starts tripping over itself with rapid-fire jokes that have a funny rate of approximately one in 10. It's not enough to distract from the fact that the supposed plot about the cavewomen is completely forgotten and then abruptly remembered several times over the course of the meandering 100-minute film.

The star-studded cast offers some moments of respite from Cera and Black's deadpan shtick, which gets old almost as soon as it begins.

Paul Rudd, who has a seemingly irrepressible ability to imbue one-dimensional slapstick characters with a modicum of empathy, appears as Abel, and the squabbling between him and the perfectly intolerable Cain (the comically-gifted David Cross) offers an amusing and surprisingly believable take on a traditional Bible story.

Christopher Mintz-Plasse, whose "McLovin" character was the best part of "Superbad," plays Abraham's son Isaac as a squeaky-voiced adolescent who is appropriately irritating and believable in a way that most characters in "Year One" are not.

The women in this film, it should be noted, are not so much characters as props. They hover on the sidelines, nubile and mostly silent, dutifully playing it straight as the wacky main characters crack jokes all around them.

In the end though, despite a few well-played scenes and several clever one-liners, "Year One" is a slow-moving slush of potty humor that has enough funny material to sustain a 15-minute stand-up routine but not a 100-minute movie.

While "Dogma" lampooned religion with irreverence and originality, and "Knocked Up" made infantile jokes appealing to a wider audience, "Year One" - which seems like it's trying to do both - succeeds in doing neither.

Everyone sure looks like they had fun making it though, and maybe that's the point: Boys will be boys. *

Produced by Judd Apatow, Claytown Townsend, Nicholas Weinstock, directed by Harold Ramis, written by Harold Ramis, Gene Stupnitsky, Lee Eisenberg, music by Theodore Shapiro, distributed by Sony Pictures.