The master of the mockumentary, Christopher Guest, is back with a new serving of his specialty, and once more it’s a treat. After serving as one of the over-the-hill rock band in Rob Reiner’s “This is Spinal Tap,” Guest went on to send up small-town theater companies in “Waiting for Guffman” and flamboyant dog contests in “Best in Show.” Now in “A Mighty Wind” he hones in on a reunion show by three folk music groups from the sixties and seventies. Though not quite up to the elevated standards of the previous efforts by Guest and his friends, it’s a charming, sometimes brilliant movie, filled with the same droll, understated humor that characterized them.

The set-up is simplicity itself. After the death of Irving Steinbloom, who managed the acts in their heyday, his son Jonathan (Bob Balaban) plans a Town Hall commemorative concert featuring three over-the-hill groups: the always-smiling New Main Street Singers (Paul Dooley, John Michael Higgins, Jane Lynch, Parker Posey, et al.), a neon-colored “neuftet” indiscriminately spreading joy; the duo of Mitch & Mickey (Eugene Levy and Catherine O’Hara), a Sonny-and-Cherish duo, who were as famous for their romance as for their music until they broke up; and The Folksmen (Guest, Michael McKean and Harry Shearer), a trio whose LPs were remarkable for lacking a hole for the spindle. Much of the film is concerned with the players’ offhandedly humorous recollections and the strains of getting together again; they’re joined by other oddballs, including Fred Willard as the NMSS’s gregarious, goofy manager; Ed Begley, Jr. as a public broadcasting honcho with a penchant for lapsing into foreign phrases; Paul Benedict as a bearded folk-music historian; Don Lake and Deborah Theaker as Irving’s two other children; and Jennifer Coolidge and Larry Miller as a dotty PR agents. The final act is the concert itself, complete with newly-created songs that, while amusingly hokey in their lyrics, also manage a surprisingly authentic feel, along with a by-now obligatory “six months later” wrap-up.

Like “Guffman” and “Show,” “Wind” is truly a joint effort by an ensemble which has become a virtual improvisational repertory company. Though the script is ascribed to Guest and Levy, it actually results from scads of rehearsals and ad-hoc sessions and more than fifty hours of footage that’s afterward been shaped and edited into a brisk hour-and-a-half. It’s a technique that the cast, by now a bunch of old friends playing off one another like a long-time vaudeville team, use to great effect. Though at times the actors go for broad, easy laughs, they more often opt for gentler, more subtle bits, and handle them deftly. (Levy, for example, earns laughter by creating an almost preternaturally subdued character.) To choose favorites is an invidious task; it’s easier to point to the bits that don’t quite come off, or at least don’t work as well as might be hoped. Those would include Willard’s boisterously stupid Mike LaFontaine, who’s not nearly as funny as his dog-show announcer in “Show”; Jim Piddock’s Leonard Crabbe (he’s Mickey’s model train-loving husband); and the home-life weirdness of the Main Street Singers’ Laurie and Terry Bohner (Lynch and Higgins), which is certainly strange but more creepy than funny. Furthermore, the big topper regarding the Folksmen, which closes the picture, is a stretch that doesn’t quite hold. But overall “A Mighty Wind” has a much higher percentage of hits than misses; and the fact that it doesn’t push too hard, quietly building and letting the gems spring out unexpectedly rather than continually italicizing its own cleverness, is a wonderful change from today’s overemphatic farces.

The result is an unforced, affectionately mocking picture which might not generate a constant gale of laughter, but does invite a steady stream of smiles and chuckles.