SOMEONE NEEDS to debunk the legend of Johnny Cash. Points of attack include Cash's shilling for Victoria Station, a chain of prime-rib joints (for sale: a plastic-wrapped mint copy of Cash's 1975 LP Destination Victoria Station). On a broadcast of his ABC variety show, Cash claimed, "I stand by the president." Meaning Nixon. Someone (Cash should have shot this someone, just to watch him die) released the early-1970s LP Johnny Cash on TV, an album "sweetened" with applause in automatic 15-second cycles.

Cruelty to steers, Nixonism and use of canned applause—accusations that will fail to mean anything in the context of the overlong but moving biopic Walk the Line. It takes the millionth walk on a familiar line—the crisis in a performer's life, resolved by marriage to a good woman.

In the lead role, Joaquin Phoenix shows Cash as a singer who nearly killed himself mixing Falstaff and Benzedrine. "You look like you're going to a funeral," says a square about the Man in Black's nighted colors: "Maybe I am," Phoenix sighs, exuding cool.

As Cash, Phoenix is subtler and softer than you'd expect, but director James Mangold (Identity) stints the performer's scariness. The black clothes weren't just his way of mourning for his own life; they were meant to advertise that he was the villain of the piece. "I'm known as bad news," Cash once sang. Someone who titles an album Mean As Hell isn't trying to be a victim of circumstances.

Phoenix's sensitivity seems a drawback when, early on, he moans "Folsom Prison Blues" instead of preaching it. Now and then, the actor adds torchiness to vocals that should be plain as pine boards. But it's a compelling, funny performance, and if you're fond of Cash it's hard not to succumb to this picture of him as a bedeviled romantic. A real star is as compelling in meltdown as he is in triumph. Phoenix's meltdown, a pill-fueled wrecking of the wry prison song "Stripes," is as good a show as anything else here.

Walk the Line commences in 1968, when Cash is standing in the wings at his Folsom Prison concert. We flash back to Cash's early years, in scenes Mangold bronzes like a baby's shoe. Mangold and co-writer Gill Dennis decide that the crisis is Freudian: Cash's father, played by the fearsome Robert Patrick, always loved Cash's brother best. Was that his trauma—rather than a system that kept Cash's family poor?

As a loner in Germany in the Air Force, Cash picks up the guitar. Back in Memphis, he supports himself and his family as a door-to-door salesman. In free time, he plays with a porch trio, including an amateur bass player who needs pieces of masking tape to guide his fingers to the chords.

When Cash takes off with his hit "Folsom Prison Blues"—a song based on a movie Cash saw, Mangold reminds us—the singer tours relentlessly, with Elvis, Jerry Lee Lewis and Roy Orbison. All performers are presented as smaller than life. It's a smart approach; they're bratty kids, like child prodigies. Tyler Hilton's Elvis only stands out because he likes to practice alone.

The road wears on Cash's marriage, in a manner that has slowed musicians' film biographies ever since the talkies began. (Ginnifer Goodwin plays the neglected wife, Vivian. Her worse line would be "I've got a casserole in the oven and my sister in the kitchen, and I don't want to know about the tour.") Meanwhile Cash is attracted to a creature more like himself: June Carter, a princess of the renowned singing Carter Family. Reese Witherspoon's June has a keen moment—as she's about to step out before an audience, she flicks on a big sunny stage smile. It happens very quickly, but we can see it. Walk the Line argues that June's problem rhymes with John's: "Junebug" considers herself the least talented of the Carter Family. If this seems a little cooked, maybe it's because Witherspoon isn't much like a performer who ever had a moment's doubt about herself.

Together, though, Phoenix and Witherspoon are heaven; it's easy to relax, to go with the illusion that you're watching the Southern lovebirds dueting on their hit "Jackson"; their own voices sound enough like the real thing to make it all work.

Walk the Line earns its slack, and more, because the music has been handled with such care. T. Bone Burnett's soundtrack may do for rockabilly what his soundtrack for O Brother, Where Art Thou? did for old-timey music. Burnett is devoted to period sound. (He has said, "To put digital reverb on a Johnny Cash song would be like putting spacesuits on the Tennessee Two.")

This soundtrack, like Mangold's movie, is not the work of dilettante fans. These people know their stuff. It's probably the first Hollywood movie that had the phrase "shape notes" in it. David J. Bomba's production design has the scrupulousness you expect in Ang Lee's movies. The country roads, bars and five-and-dimes are tremendously nostalgic. Walk the Line is made up of the juicy incidents most biopics ignore. The Lincoln of country music may have been a man of sorrows, but Walk the Line is exhilarating, a burst of triumphant Americana.

Walk the Line (PG-13; 136 min.), directed by James Mangold, written by Mangold and Gill Dennis, photographed by Phedon Papamichael and starring Joaquin Phoenix and Reese Witherspoon, opens Friday valleywide.

Send a letter to the editor about this story.