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. Who would want to be so judgmental and shallow, or so blissed-out and dim? Not Megan. But then again, it isn’t as if her own life were anything fabulous.

Megan has been trained as a therapist, but she’d rather work for her father (Jeff Garlin), an accountant, dancing around with a sign to attract customers. Although she feels free to drop by her parents’ house to watch TV and score a free meal whenever she likes, she lives with her high school boyfriend, a wet noodle named Anthony (Mark Webber), who has finally worked up enough courage to suggest marriage. His fumbling proposal, along with some other stuff, precipitates the central crisis of "Laggies," Lynn Shelton’s curious, intermittently winning hybrid of coming-of-age story and romantic comedy.

Like Shelton’s earlier films ("Your Sister’s Sister" and "Humpday"), this one takes place in and around Seattle, a place apparently full of unpretentious white people, neither intimidatingly cool nor tediously generic. Knightley, a proven chameleon, fits right in, suppressing her British accent, showing off her skateboard moves and even dabbling in a bit of vocal fry. She makes Megan perfectly likable, which isn’t entirely a good thing.

‘LAGGIES’ Rated: R Opens Friday at Kahala 8

"Laggies" itself is so eager to be liked that it undermines some of its comic ingenuity. Like the confused guy played by Mark Duplass in "Your Sister’s Sister," Megan stumbles into a situation that is inappropriate in ways she doesn’t quite understand. Fleeing her best friend’s wedding, she winds up hanging out with a group of teenagers who seem like a more comfortable fit than the old crowd. Later, fleeing from Anthony, she crashes on the bedroom floor of one of them, a savvy and self-possessed girl named Annika, played by Chloe Grace Moretz with her usual poise and discipline.

Annika lives with her sarcastic, heartbroken dad, Craig (Sam Rockwell), her mother (Gretchen Mol) having run off some time ago to become a lingerie model. Megan assumes a curious role in the household. She’s Annika’s confidante but also a surrogate big sister and almost-mom, toggling between silliness and wisdom and meanwhile bantering and sidestepping toward a romance with Craig.

There is enormous potential here for misunderstandings, bad feelings and serious trouble — the kind of friction that could generate either rattling melodrama or giddy farce. But Shelton sticks to a safe and dubious middle ground, making sure to resolve conflicts before they become too messy and encouraging everyone to learn the necessary lessons as quickly and painlessly as possible. You might think that a vulnerable 16-year-old girl might be a little upset, or mildly creeped out, if her father and her new best friend started sleeping together. "Laggies" allows you to think that for about half a minute, before shrugging, smiling and moving on.

This is a nice movie. It’s frisky and cheerful, even when tears are on the way. But it isn’t a very good movie, mainly because, like its heroine, it’s reluctant to make up its mind about what it wants to be. The rough, lived-in, naturalistic look belies a soft sitcom heart, and every foray into risky emotional territory prefigures a retreat into cliche. It turns out that Megan, rather than finally negotiating a passage to independence, is just moving from one dude to another, trading in a needy man-child for a daddy.

Review by A.O. Scott, New York Times

© 2014 The New York Times Company