In recording-industry terms, the Keira Knightley/Mark Ruffalo musical romance "Begin Again" is a Difficult Second Album. Irish writer/director John Carney scored a big hit with his debut film, the sweetly melancholy 2007 indie love story "Once." It transported audiences, won an Oscar and inspired a smash Broadway spinoff.

Try following a genius opening shot like that.

"Begin Again" closely parallels the mismatched-partners design of "Once" while adding new chords to the arrangement. Down-on-his-luck music exec Ruffalo hasn't signed a strong act in years, and his personal life is dumpster-worthy. He's a fog of disappointments held together by Scotch and sarcasm.

At a Lower East Side bar he sees Knightley strumming a smart, plaintive love song. Imagination afire, he visualizes the unmanned piano, drums and electric bass playing along. His dream team sounds so fine the corny invisible-man combo works.

Ruffalo reaches out to her with a business proposal that may be something more. He wants to sign her; she's not interested. She's heading back to England, having broken up with her suddenly successful musician boyfriend. Maroon 5's Adam Levine looks quite at home in the douchey role.

Ruffalo needs Knightley to resuscitate his career. She needs him for his pushy encouragement and a recording project to take her mind off Levine. The laws of movie magnetism usually decree that a film's stars will fall in love. Knightley and Ruffalo are such an unlikely package that seems like a long shot.

The actors themselves look unsure where their relationship is headed. Knightley brings emotional confusion into sharp, touching focus as the pair bond over nighttime strolls and the shared love of classic pop. Carney flips film clichés to keep us guessing.

"Begin Again" improves on the typical genre picture with its rueful intelligence, warmth and creative use of music to advance the story. The breakup song Knightley leaves on Levine's voice mail is a stinging dope slap of a song (Gregg Alexander of New Radicals composed the original tunes). Knightley's album project, recorded live in colorful/seedy outdoor locations, makes the city a supporting player. In alleys, on rooftops, in Central Park rowboats — you never know who'll object to the jamming, or unexpectedly join in.

There's a fine roster of music stars in the fold, with Yasiin Bey (aka Mos Def) as Ruffalo's cool as a cucumber business partner, and CeeLo Green as a vastly successful early client.

Some viewers might feel cheated that they don't perform and Knightley does. I found her acoustic guitar and breathy vocal work sweet and apt for a songwriter. Some of the songs may drift away, but the acting will stick in your head for days.