Parody only works with sincerity, and there's a mighty wind of both in this faux life story of a musical legend who's part Johnny Cash, part Brian Wilson and part every wand'ring troubadour who goes out in the world with nothing to offer 'cept a dream and some rhymes he wrote 'tween here and Tupelo... Sorry, sorry—it's just that when Walk Hard: The Dewey Cox Story rubs off on you, you'll never be able to watch VH1's "Behind the Music" or probably Walk the Line, The Buddy Holly Story or Great Balls of Fire ever again without laughing your great balls of fire off.

No disrespect meant to Cash, Holly or those other early rockers who handcrafted a musical form. You have to have affection, after all, for whatever you're parodying, which is why there are very few parodies of, oh, plumbing-supply magazines. Walk Hard is a pitch-perfect parody of pop-music biopics, not only because it mimics the form like a rock ’n’ roll replicant, but because it loves the archetypal subjects so much that it feels comfortable enough to kid them about their excesses and striking similarities. Director Jake Kasdan, who co-wrote with modern comedy's very busy man-of-the-moment Judd Apatow, hits every note just right.

Helping in no small measure, so to speak, is star John C. Reilly, who besides pulling off a range of melodramatic emotions with just enough anguished reality (to his disapproving wife: "I think I'm doing OK for a 15-year-old with a wife and a baby!"), helped write several of the numbers, and shivers his timbre to capture everything from Buddy Holly chirps to Bob Dylan nasality to Donovan/John Sebastian hippy trippiness to The Beatles in their Maharishi phase. (No less a cult-fave than Marshall Crenshaw co-wrote the title song, with Reilly, Apatow and Kasdan.)

The story opens on a farm down South, as of course it must. After a childhood tragedy involving his younger brother Nate, young Dewey goes "smell-blind" from the trauma. Worse, he must carry the guilt for the prodigious Nate's marvelously and repeatedly foreshadowed death. A resentful Pa (Raymond J. Barry) keeps muttering how "the wrong kid died," and though Dewey's mama (Margo Martindale) loves him, Dewey must set out at 14 to make his mark on the world, taking with him his young wife Edith, 12. The allusion to Jerry Lee Lewis' child-bride isn't at all creepy since in this scene and forward, Dewey and Edith are played by Reilly and by "Saturday Night Live"'s Kristen Wiig with no attempt at making them look younger than their respective forty-something and thirty-something selves.

You can chart Dewey's rise, fall and redemptive state of grace in his golden years, as he sorta Forrest Gumps his way though annotated history—telling Buddy Holly (Frankie Muniz in a cameo), "Man, I'm awful nervous, Buddy Holly." Lifelong love Darlene Madison (an eye-openingly dishy Jenna Fischer) eventually reenters his life with, "It's me, Darlene, at age 50." If you know a bit of pop-music history, then the attempted-magnum-opus scene in a recording studio with full orchestra, battalions of backup singers and every ethnic instrument on Earth will make you howl like you're having a Spinal Tap.

As with that seminal faux band, Dewey's songs are dead-on—from the wistful ’50s "Take My Hand" to the innocently dirty Cash-Carter pastiche "Let's Duet" ("In my dreams you're blowing me / Some kisses") to the protest song "Let Me Hold You (Little Man).” We could have done without the three brief shots of full-frontal flaccidity, but then you have something as great as a virtually unrecognizable Paul Rudd, Jack Black, Justin Long and Jason Schwartzman in a two-minute cameo as John, Paul, George and Ringo leading up to Yellow Submarine-type animation and, and, and...look, don't walk hard, run to see this. I'm sure I won't be the only one using those very words.