Ten years after senior prom, Megan (Keira Knightley) finds herself in limbo, no longer adolescent and not yet fully grown. It’s a familiar place for the protagonist of a movie comedy to be, and perhaps a further symptom of the shaky state of American adulthood. In Megan’s circle of high school friends, the one who is about to get married (Ellie Kemper) and the one who already has a husband and a kid on the way (Sara Coates) seem kind of awful.