You can watch the trailer right here or in HD over at Apple.

Here's the official synopsis:

MARGARET centers on a 17-year-old New York City high-school student who feels certain that she inadvertently played a role in a traffic accident that has claimed a woman’s life. In her attempts to set things right she meets with opposition at every step. Torn apart with frustration, she begins emotionally brutalizing her family, her friends, her teachers, and most of all, herself. She has been confronted quite unexpectedly with a basic truth: that her youthful ideals are on a collision course against the realities and compromises of the adult world.

Kenneth Lonergan, who contributed to the scripts for Gangs of New York and Analyze This, made an auspicious directorial debut more than a decade ago with You Can Count On Me, which featured amazing performances from Ruffalo and Laura Linney.  An extremely drawn-out editing process and lawsuits over completion funding for the film have made Margaret somewhat notorious, as production took place all of six years ago.  That helps explain the ten-year gap between Lonergan's films.

Anyways, the cast includes a bevy of hugely talented actors, including Jean Reno, Olivia Thirlby, Kieran Culkin, Allison Janney, Krysten Ritter, Rosemarie Dewitt, and Matthew Broderick, who reportedly loaned Lonergan the money to finish editing.

Margaret finally arrives in theaters on September 30th.