There are many reasons why "Begin Again" continues to have such great staying power against the big guns this summer. Expanding its run in L.A. this weekend, the story is a catchy one, the music is great, and it is flavored by real musicians, including Mos Def, CeeLo Green and a star turn by Maroon 5's Adam Levine. Irish filmmaker John Carney, the writer-director whose indie sensation about a Dublin street busker, "Once," became a Tony-winning show (now playing at the Pantages Theatre in Hollywood), is a lover of musicians and their flaws. Those passions seep into his work in mysterious and mesmerizing ways. "Begin Again" is New York street through and through, with a link to London via its undiscovered singer-songwriter Gretta (Keira Knightley), who's come to the city with her soon-to-be-rock-star boyfriend, played by Levine. Perhaps because we've meet Knightley's Gretta at a fragile moment in her life, love lost, all that, the actress shows a lovely softness we rarely see. Mark Ruffalo is perfectly bruised as the broken-down record producer in the midst of a bender when he discovers Gretta. The secret to the film's success is the way it lets the music — and the making of it — mend any broken heart in sight.