James Gardiner stars in Olney Theatre Center’s production of “Little Shop of Horrors.” (Stan Barouh/Courtesy of Olney Theatre Center)

“Fun” was the word that book and lyrics writer Howard Ashman used to describe his goal, with composer Alan Menken, for the man-eating plant-from-outer-space musical “Little Shop of Horrors.” On what planet could anyone say that they failed?

This, in case you missed the amusing 1986 movie with Rick Moranis and Steve Martin or the professional and school productions that still pop up like weeds, is the show with that ravenous plant crooning “Feed me all night long” with Temptations-style danger. It’s also the one with the scene-stealing sicko biker dude jubilantly singing “Be a Dentist” (which, in fact, he is), and with a Motown girl group shoop-shooping their own wise commentary all the way through. Fun, fun, fun!

Yet “Little Shop,” which opened off Broadway in 1982 and stayed there for more than 2,000 performances, is currently wilting at the Olney Theatre Center. Nobody seems to be enjoying the joke except James Gardiner, whose bashful cockeyed grin and cheery singing as the nerdy floral shop assistant Seymour (who learns to nurse the mysterious monster plant with blood pricked from his finger) gives the show nearly all the pep it gets.

It’s oddly earnest, this dim and measured staging by Mark Waldrop. Everything is cautiously functional, from James Fouchard’s drab fake-brick set for the Skid Row flower shop to David Kaley’s only slight tacky costumes for Seymour and Audrey, the bruised and mousy beauty he adores. It’s almost as if Waldrop actually believes the story and wants us to treat it as real.

The humongous plant puppet singing and gobbling up the cast (this is ultimately a sci-fi saga of intergalactic domination) tells you otherwise, of course. But so does the music: “Little Shop” is an exceptionally witty goof cooked up by gifted songwriters in their salad days. Ashman, who died in 1991, went on to write Disney’s “Little Mermaid” and “Beauty and the Beast” with Menken, and they spoofed 1960s music with real affection here. The catchy choral “Skid Row,” the gloriously silly love duet “Suddenly Seymour,” and those shang-a-lang-lang phrases from the ever-present girl group: that’s the heart and soul of the show.

Yet music is hardly the strength of the Olney’s production. The small pit band is muffled and the singers are often overbearingly overmiked; if the Olney is going to continue producing a couple musicals a year, someone ought to figure out how to create alluring sound each time out. Nobody moves with verve, either, despite the dance-ready beats of Menken’s score.

So numbers that ought to soar just go phhht. Bobby Smith, one of the savviest musical actors in town, is stranded as the sadist dentist during his two songs, “Be a Dentist” and the particularly awkward “Now (It’s Just the Gas).” Carolyn Agan, breathy and delicate as many an Audrey before her, is guided to take Audrey’s puckish ballad “Somewhere That’s Green” at such a lugubrious tempo that a kid in the back row could be heard whispering during a pause, “I don’t like it.”

Ford’s Theatre ran into the same sort of wall with “Little Shop” two years ago; the production somehow seemed preoccupied with the meaning of the show’s story. In a musical, though, especially the fun ones, the story is an excuse for the crafty songs — not the other way around.

Little Shop of Horrors

Book and lyrics by Howard Ashman, music by Alan Menken. Directed by Mark Waldrop. Musical director, Ross Scott Rawlings. Choreographer, Vince Pesce. Lights, Charlie Morrison; sound design, Jeffrey Dorfman. With Kara-Tameika Watkins, Shaunte Tabb, Leayne C. Freeman, Ethan Watermeier, Stephawn Stephens, Tina Ghandchilar, Matthew Schleigh, and Russell Sunday. About two hours. Through Aug. 26 at the Olney Theatre Center, 2001 Olney-Sandy Spring Rd., Olney. Call 301-924-3400 or visit olneytheatre.org.