One really wants to like “Matthew Modine Saves the Alpacas,” a world premiere play by Blair Singer (“Weeds”). Modine seems a good sport to play a washed-up version of himself aching to rejoin the A-list, and the notion of his vaulting back to stardom via a self-aggrandizing do-gooder cause is promising. But a premise is all they’ve got as the evening declines into snoozeworthiness. Most TV series take three seasons to jump the shark, but in the theater it can happen in 20 minutes, and so it goes at the Geffen.

There’s gleeful schadenfreude in an ’80s movie star living greasy-haired and sparkin’ fatties in a Winnebago, now prostrate at the feet of bigtime publicist Whimberly North (stylish Peri Gilpin of “Frasier”). When the Chimborazzi tribesmen of Ecuador are threatened with extinction because their alpacas are dying, a plan emerges: Modine to the rescue.

Portrayed as the biggest numbnuts on the planet, Modine does “Modine” with convincingly clueless desperation. Gilpin is sharp and sexy, and it’s not her fault the Hollywood Dragon Lady has already been played to a turn (with better jokes) in “The Little Dog Laughed.” The stage is set for laughs.

Yet even then, doubts about the farcical logic emerge. Most celebs associated with world causes are already at fame’s pinnacle; would the process really work in reverse? Exactly why will the tribe die off if its alpacas disappear? (That’s the plot’s engine, and the characters keep repeating it, but it’s never explained.) Shouldn’t someone suggest the obvious remedy of importing alpacas from the outside, if only to shoot the option down?

And why does it take so unconscionably long for Whimberly to accept Matthew as a client? Since she has to do so or there’s no play, are they simply marking time because they’re running out of story?

The play’s shift to the Andes confirms one’s fears. No one has worked out whether the Chimborazzi are to be caricatured or taken straight, so there’s a little of each: They bumble along with Moe Howard hairdos and phony accents, but Santos (Mark Damon Espinoza) is painted as a serious villain who would rather see his tribe die out than accept Modine’s help — a dubious stance even by farce standards.

Our hero arrives to help the village, where a close encounter with alpaca poop threatens to divert this rabid germophobe’s vision quest. (How a germophobe could survive in a filthy trailer is another mystery.) But out comes the Purell and in come Matthew’s wall-to-wall hut carpeting and plans to dredge the local lake, mentioned two or three times but not funny once.

Now it’s clear they’re stretching what could’ve been an acceptable Carol Burnett sketch into two hours, the broad playing and Beowulf Boritt’s painted flats certainly exuding a ’70s variety-show ambience.

Increasingly frenzied, witless action hits rock bottom with the introduction of a U.N. rep (French Stewart), a pencil-mustached Gallic loon with Inspector Clouseau accent, lame malapropisms (he calls Santos “Satchmo”) and zero plot connection. Stewart (“3rd Rock From the Sun”) is sleekly amusing as Whimberly’s gay assistant but insufferable here.

Helmer John Rando (“Urinetown”) has been known to work comic magic but can’t make a silk purse out of an alpaca’s ear: A cadre of puppets appear to little purpose, unless seeing the violation of a creature resembling Lamb Chop is your idea of fun.

Matthew Modine Saves the Alpacas

Geffen Playhouse, Los Angeles; 522 seats; $75 top

  • Production: A Geffen Playhouse presentation of a play in two acts by Blair Singer. Directed by John Rando.
  • Crew: Sets, Beowulf Boritt; costumes, Robert Blackman; lighting, Jeff Croiter; sound, Jon Gottlieb; fight direction, Steve Rankin; production stage manager, James T. McDermott. Opened, reviewed Sept. 16, 2009. Runs through Oct. 18. Running time: 2 HOURS.
  • Cast: Matthew Modine - Matthew Modine Whimberly North - Peri Gilpin Jeffrey, Pierre du Perrier Jouet - French Stewart Abraham - Edward Padilla Santos Panchos, Puppeteer - Mark Damon Espinosa Angel - Reggie De Leon <b>With:</b> Mark Fite, Lauren Lovett, Kevin Noonchester.