Radio is a lonely, romantic business. A tower, somewhere out on the prairie, broadcasts words and sounds and music and traffic and weather into the emptiness, and perhaps somewhere a few souls taking a rest from beating a living out of the earth tune a broken-down old contraption (do they even still make radios anymore?) in to pick up the fading signal.

At least, such is life in the world of “A Prairie Home Companion,” the movie version of Garrison Keillor’s beloved radio show of the same name. Directed by Robert Altman, and with a screenplay written by Mr. Keillor himself, the film reimagines the National Public Radio staple as an obscure – and doomed – transmission received by only a few hundred local fans. With an easygoingness bordering on narcolepsy, “Prairie” is a lazy but absorbing pleasure, especially if you’re a fan of folk music and droll Midwestern wit. But its structure is perplexing, with tangents going nowhere, star cast members wasted (or seriously miscast), and a central conceit that at times seems like an afterthought.

First, that conceit. The film’s action centers around Mr. Keillor, playing himself (the role he was born to play, quite seriously), leading the gang from his old-timey radio variety show through what they learn at the beginning is to be their final performance.The tiny station they call home, it turns out, is being swallowed up by a Texas conglomerate, run by “people of faith,” that considers the show, and everyone connected to it, an anachronism – charming perhaps, but of no great interest, profit-wise.

Putting aside that lethargic swat at the current presidential administration, there doesn’t seem to be much reason for the “last show” story device. In fact, what we see instead is essentially a backstage look at a run-of-the-mill edition of the show, filled with Mr. Keillor’s trademark fictional ads for products such as duct tape and “Powdermilk Biscuits” (“in the big blue box with the picture of the biscuit on the cover”), folk music acts like the Johnson sisters (with Meryl Streep and Lily Tomlin playing the remaining members of a once-promising family singing troupe done in by an incident involving a doughnut), and cowboys Dusty and Lefty, played by Woody Harrelson and John C. Reilly, trading bad jokes about “soup or sex” and Viagra.

It’s when the simple joy of watching the show unfold is unimpeded that “Prairie” is at its strongest. Just watching Mr. Keillor deliver some of his standard bits is a treat, and the airier backstage antics are quite fun. Lindsay Lohan, in particular, turns in a charming performance as Ms. Streep’s suicide-obsessed daughter. The interplay between shock-seeking teenager and utterly uncomprehending Midwestern mother provides some of the funniest moments in the film.

But when Messrs. Keillor and Altman turn to the superfluous “plot,” things fall apart.To keep things moving, Guy Noir – a regular character in a sketch on the real-life show, a hard-boiled detective a little soft in the head – is brought to life by Kevin Kline as the theater’s bumbling security guard to narrate events. Brought to life, though, might be too strong a term, as Mr. Kline is unable to ditch his own grating persona long enough to do the character justice.

Virginia Madsen is lovely as a blond angel of death, but just what she’s doing in the movie is a mystery. Similarly, Tommy Lee Jones shows up late in the film as the Texas company’s “Axeman,” but it’s hard to imagine why he took a role where his character does … absolutely nothing.

Worst of all, Maya Rudolph, one of the few bright lights on “Saturday Night Live” these days, is cast as a very pregnant assistant stage manager (she was very pregnant in real life at the time) – but the role has so little meat on its bones it might as well have been tossed to an extra.

Death hangs heavy over the entirety of “A Prairie Home Companion,” and not just because there’s a comely angel of death running around. As screenwriter, Mr. Keillor has penned a eulogy for the golden age of radio, bemoaning that all that’s left on the dial these days is people yelling and computers playing music. As a character, however, Mr. Keillor refuses to turn his last show into a eulogy (even after a performer expires backstage), sermonizing to his loyal crew that, when it comes down to it, every show is your last show.You’ve just got to go out and do your job every day.

Yet, there is something romantic about mourning the death of radio, even as innovations such as satellite radio and podcasting offer hope of a renaissance for the medium. Perhaps it’s just as simple as the pride one takes in being the only one, or at least one of a select few, to appreciate something truly special.

For all its flaws, Messrs. Keillor and Altman’s movie leaves the audience with that feeling of being in on something good. And it also leaves one agreeing with Ms. Streep’s character, who declares at the end of the movie that she wishes she could just keep playing last show after last show.