This cute, sugary musical is the handiwork of Stuart Murdoch, singer-songwriter for the Scottish soft-pop band Belle and Sebastian, and though he’s in his mid-40s, his movie seems to be aimed squarely at moony 19-year-olds. The mousy, anorexic heroine (Emily Browning) prematurely checks out of a mental hospital to go larking about with a wimpy, bespectacled guitarist (Olly Alexander), and along with his pretty, puckish girl pal (Hannah Murray), they instantly recruit a rhythm section and start playing fun shows that never involve dragging equipment around or trying to get paid. Like the Belle and Sebastian records, Murdoch’s songs here are sprightly and sophisticated, and the musical numbers are colorfully, exuberantly staged; if only the characters weren’t so twee and self-pitying, he might really have had something here. The movie is like a milk shake so sweet that you want to keep drinking, even as you begin to gag.