You can be the scrappy newcomer only, well, once. That's a problem for Once writer-director John Carney, who has refashioned his low-budget 2006 hit as the slicker, cornier Begin Again. The new film excels as a pop-music fairy tale, but its real-world notes are seriously off-key.

The movie originally traveled the film-fest circuit under an unfortunate title, Can A Song Save Your Life? As in Carney's earlier effort, the life to be saved is that of a struggling man, and the rescuer is a young woman. This time, though, the intimacy is entirely musical.

The story opens at a New York open-mic session that the gently curling narrative will subsequently revisit twice. On stage is melancholy, fragile-voiced Greta (Keira Knightley, who's actually singing). Aside from her busker pal (James Corden), no one's digging the performance. Enter Dan (Mark Ruffalo), a booze-marinated music-biz lifer who's lost his wife, his daughter and — just today — his job.

After a rush of flashbacks to explain Dan's plight, the movie returns to the club. In a charming sequence Once's limited financing couldn't have covered, Dan imagines a full-band treatment for Greta's Suzanne Vega-like lament. As he fantasizes, the unused instruments on stage play themselves.

Do you believe in magic? There's a lot of it in Begin Again, and it's more cogent than the stuff that's meant to be gritty.

Dan wants Greta to be his comeback project, but she's reluctant. The British songwriter traveled to New York with longtime boyfriend Dave (Maroon 5's Adam Levine). He signed a lucrative recording contract and promptly cheated on her; she's already booked her ticket home.

After some cajoling and much bonding — notably over their remarkably mundane favorite tunes of all time — Greta and Dan agree to make an album together. They're bankrolled by one of Dan's former discoveries, a now-rich hip-hopper (Cee Lo Green). The insurgent popster and her producer decide to record on streets and rooftops. In a move worthy of a Mickey Rooney/Judy Garland vehicle, they incorporate local urchins into their eclectic but always honeyed arrangements.

Carney used to be a member of the Frames, the Irish pop-folk band fronted by Glen Hansard, Once's leading man. To judge from Begin Again, he's lost touch with the music business. Among the many howlers is a scene with Dan's former partner (Yasiin Bey, a/k/a rapper Mos Def). He announces that he likes the album, but that some of the songs — recorded live with no separation of instruments — need to be remixed.

The movie's tunes do need something. Mostly written by song doctor Gregg Alexander (formerly of the New Radicals), Greta's repertoire features winsome melodies and cliche-choked lyrics. Even the angry ditty she sings to Dave's answering machine is trite.

Greta may not be the new Joni Mitchell, but she becomes a sort of Good Witch of the West Village. She not only revives Dan's career, but also sparks his reconciliation with estranged wife Miriam (Catherine Keener) and bitter teenage daughter Violet (Hailee Steinfeld).

This would be preposterous if not for the vivacious Knightley, an irresistibly watchable force of nature ever since Bend It Like Beckham. The music industry, like girls' soccer, can't help but be overwhelmed.

Benevolent Greta teaches Violet about boys and clothes, and even enlists her to play a guitar solo. The girl holds back for nearly the whole song, apparently intimidated, but then just in time picks up her guitar and nails it. It's a moment that, like the movie itself, is both laughable and sweet.

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