The Aldous factor.

THERE'S only one reason to see Forgetting Sarah Marshall, and his name is Aldous Snow. Or maybe I should say Russell Brand, since the line between the actor (Brand, that is) and the character he plays is not exactly clear. Snow, the lead singer of a band called Infant Sorrow, is the musician for whom the Sarah Marshall of the title dumps the movie's hero, and he's a vain and utterly hilarious popinjay--a long-haired, hip-waggling, eyeliner-sporting, dead-on parody of dozens of interchangeably self-absorbed pop stars.

The real-life Brand, meanwhile-a British celeb just beginning his inevitable invasion of our shores--is, well, a vain and hilarious popinjay, whose entire career looks like a long send-up of the famous-for-being-famous crowd. Floated on little more than charm, outrageous outfits, and a flair for deadpan one-liners, he's made his way as a radio and television host, a newspaper columnist (British papers are rather more fun than ours), a sketch comedian, and the author of a tell-all memoir about his various addictions (sex, drugs, etc.) titled My Booky Wook. His turn as Snow, then, isn't exactly a stretch, but it's a worthy addition to his resume, and I would almost suggest buying a ticket to Forgetting Sarah Marshall just to see him perform Infant Sorrow's hit single "We've Got to Do Something," a glorious parody of the "We Are the World" school of politically engaged pop music.

Fortunately, though, you can probably find the video on YouTube and spare yourself the film, which is unworthy of both his gifts and your attention. Marshall is the latest production from the Judd Apatow comedy factory, which has turned out delights like The 40-Year-Old Virgin and last year's Superbad, but which of late seems to be entering the period of rapid decline that all comedic posses eventually fall into. It's a sociological problem as much as an artistic one: When you hit it as big as Apatow has, the natural thing to do is help your friends and associates (and their friends and associates, and so on) hit it big as well. This means, at first, giving breaks to people who richly deserve them; then to people who sort of deserve them; and finally to people who don't really deserve them at all.

Thus the mediocrity of last Christmas's Walk Hard and the awfulness of March's Drillbit Taylor. Thus, too, Forgetting Sarah Marshall, a forgettable (sorry, couldn't help it) serving of comedy written by and starring Jason Segel, and directed by Nicholas Stoller--neither of whom, unfortunately, is up to the task of carrying off even as lightweight a piece of cinema as this. Segel has done solid supporting work for the Apatow machine in the past--in last summer's Knocked Up and the short-lived TV shows Freaks and Geeks and Undeclared-but as a leading man he's charmless, flat in dramatic moments and overbroad in comic ones. And he's poorly served both by his own work as a writer (at least in the non-Aldous Snow portions of the script) and by Stoller's direction, which is by turns choppy and inert.

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The movie's story is a variation of a well-worn Apatowian theme, in which a man-child has to emerge from his cocoon and face the realities of grown-up life. In earlier, better films, these cocoons have ranged from the pre-sexual comforts of a high-school friendship (Superbad) to the coziness of slacker adultescence (Knocked Up) to the dubious security of perpetual celibacy (The 40-Year-Old Virgin). Here, it's the long-term relationship Segel's Peter Bretter enjoys with Sarah Marshall (Kristen Bell), who stars in the CSI-style television drama for which he works as a composer.

Their years together have turned Bretter into a professional wallflower and a cereal-munching shlub, but he thinks the relationship is the best thing that's ever happened to him, and when she dumps him he's devastated beyond all comprehension. (The dumping scene occurs just after he's emerged from the shower, and Segel plays the scene entirely--and I do mean entirely--in the nude, which is funny, though not quite as funny as the movie thinks it is.) Cue several months of drinking, debauchery, and depression, followed by an impulsive trip to a Hawaiian resort-the same resort where, inevitably, none other than Sarah Marshall is vacationing with her rocker beau.

It's a first-rate setup, but too much else is second-rate--not only Segel's performance and his script, but Bell's one-dimensional turn as his ex, Mila Kunis's effort as the hotel employee Bretter ends up falling for, and most of the performances around them. The female leads feel smaller-than-life, like TV stars elevated above their station, which is exactly what they are: Bell cut her teeth on Veronica Mars and Kunis on That '70s Show, and both seem best equipped for work that keeps them on a screen measured in inches rather than feet. The supporting cast includes a cameo by Paul Rudd, an Apatowian who deserves all the success that comes his way, which lends a touch of class to the proceedings. But everyone else is either mediocre (Jonah Hill as an obnoxious waiter, Bill Hader as Bretter's straitlaced sibling, several heavyset Hawaiians as, well, heavyset Hawaiian stock characters) or, in the case of the pair of sexually challenged Christian newlyweds whose struggles take up vastly too much screen time, unforgivably unfunny.

Only Aldous, hilarious Aldous, makes the movie more than a chore to sit through. See it for him, or don't see it at all.