Margaret (15)

Ten years ago Kenneth Lonergan made a lovely and heartbreaking feature debut called You Can Count On Me. I wondered from time to time what this writer-director would do next, and how he'd cope with the burden of expectation.

We now have the answer in Margaret, and I half-wish we didn't. It is an intermittently enjoyable but overlong and uneven film. On a scene-by-scene basis the first hour works pretty well, sketching the portrait of a Manhattan high-school student named Lisa (Anna Paquin) whose comfortable Upper West Side life is jolted when she is witness to (and partial cause of) a woman's death in a bus accident. You Can Count On Me also began with a fatal car-crash, so the omens look good.

Lisa is bright, articulate, combative and far more insecure than she lets on. What impresses is Paquin's utter commitment to the role, however insufferable the character will become. Lonergan surrounds her with some serious talent – J Smith-Cameron terrific as her actress mum, Matthew Broderick and Matt Damon as her two schoolteachers, Jean Reno as her mum's shy suitor, Mark Ruffalo as the bus driver – and Paquin holds her own with all of them. What Lonergan hasn't provided is a focused and properly worked-out plot. Its central thread involves Lisa's relationship with Emily (Jeannie Berlin), the dead woman's spiky best friend, and their concerted attempt to publish the truth of that accident. But along the way it takes in post-9/11 terrorism and Islam, child-of-divorced-parents antagonism, inappropriate sex, and a depressing glimpse into the US justice system. It's a honking traffic-jam of issues.

The movie was actually shot in 2005 and thereafter ran into editing troubles that have plainly not been resolved. Lonergan creates some very funny scenes – like a Shakespeare class in which a single line from Lear gets Matthew Broderick into a pickle – but can't shape them into a coherent whole. At one point it's suggested that Lisa has battened on the tragedy as a means of working out in her own "moral gymnasium" (the coinage is from Shaw), though the charge of unassimilated issue-mongering could just as easily apply to Lonergan.

In the last 40 minutes he lets the movie run away from him completely as one character is killed off, more tantrums thrown and a rough truce negotiated. If you're wondering who Margaret is, by the way, she's mentioned in a passing quotation from a Gerard Manley Hopkins poem ("It is Margaret you mourn for"). But you wouldn't be surprised if you heard she was a character who got lost in the movie's haphazard composition.