If ever anyone wants to know the essence of American greatness - the warmth, the grace, the gentle wit, the insistence on salvaging every possible memory of one's life - the greatness that lies hidden beneath today's surface of national bigotry, prejudice, hubris and corruption, be sure to show them "A Prairie Home Companion," Robert Altman's film of the magic engendered every week on radio by Garrison Keillor. Certainly all immigrants, legal and illegal, should see this film to better understand why they intuitively chose the United States instead of, say, Denmark or Argentina. This film says that there is still hope for us, that George W. Bush too shall pass.

"A Prairie Home Companion," whose screenplay was written by Mr. Keillor, takes as its conceit that the Fitzgerald Theatre in St. Paul (named for that famous son of the city F. Scott Fitzgerald) has been bought by a Texas conglomerate, in the person of Tommy Lee Jones. It will be torn down, turned into a parking lot, and this - tonight's show - will be the last one ever. The film unfolds in almost real time, from just before the opening until the closing song. In addition to some of the regulars currently heard on the show, Altman has assembled an unlikely but absolutely delicious cast: Meryl Streep and Lily Tomlin as the two remaining sisters of what was once a four-sister singing act - "Just like the Carter family, except they were famous" - along with Lindsay Lohan as Streep's daughter, a poet obsessed with suicide. And Woody Harrelson and John C. Reilly as Dusty and Lefty, two cowboy singers who tell wonderfully risqué jokes between songs. Less successfully, the film turns private eye Guy Noir from an episodic and thoroughly fictional character into the real Kevin Kline, former private eye, now guardian of the theatre's stage door. It also adds Virginia Madsen as an angel of death. Kline can't seem to find a way to be Guy rather than play him; Madsen, on the other hand, is the most likeable angel of death since Emma Thompson in "Angels in America."

It's obvious that Altman hasn't been this relaxed in many years; his camera just happens to find itself in the right place at the right time, every time. We are in the Fitzgerald Theatre, we are backstage, we are onstage, we are in the family of the show, we are in the audience for the performers. Although Keillor has foregone his Lake Wobegone monologue, he is very much the same onstage persona we've known all these years. And giving us an unexpected glimpse into an off-mike Keillor, he shows us a cold, uninvolved, emotionally blank man consumed by that radio persona. Of course it would be hard for him to confront the death of his show, even in a movie, but without a flicker of emotion he leaves an empty core in the middle of the film. Even Streep, in a marvelous aside, points this out to him as the reason their own short affair never lasted.

But this is just being picky; "A Prairie Home Companion" is a rare chance to regain some of the lost qualities we used to have.