PARK CITY - Lynn Shelton has brought four of her films to Sundance, and I've been here for all four of them. Before now, "Your Sister's Sister" was my favorite of her films, while last year's "Touchy Feely" was the one with the most problems. She has rebounded in fine form with "Laggies," the first film she's directed from a script someone else wrote, and I suspect she's looking at her first possible cross-over hit here, due in part to the winning ensemble she put together.

Andrea Seigel's script is one of those tricky pieces of screenwriting where the wrong tone or the wrong cast could have sunk the film completely. Shelton's always had a strong rapport with her actors, though, and she cast this one perfectly. I feel at this point like I owe Keira Knightley some sort of apology. It has taken me a long time to connect to her as a performer, and in some of her early films, she is the ingredient that actively pulled me out. When I saw "Can A Song Save Your Life?" at Toronto last year, I found her enormously winning, though, and in this film, she gives a very smart, deeply felt performance, and she owns the film from start to finish.

Megan (Knightley) has been with her boyfriend (Mark Webber, pouring on the sincerity to the point of absurdity) since high school, and she's got all the same friends she had back then as well. When their 10th anniversary high school reunion rolls around, everyone celebrates together, but Megan finds herself facing an uncertain future, and she's not sure if she feels like celebrating anything. She loves to let her father (Jeff Garlin) treat her like a kid, and she seems to be actively avoiding the use of her degree in family therapy.

One night, after witnessing something that upsets her deeply, she finds herself outside a grocery store where she meets a group of kids including Annika (Chloe Grace Moretz), and they ask her to buy them some booze. She goes one step beyond that and actually hangs out with them for the rest of the night to drink it and to take a break from the life she's starting to feel smothered by. She finds herself really drawn to Annika, and she ends up telling her boyfriend that she's going to go to a self-help seminar he's been suggesting for a week. Instead, she's just running away to consider his just-sprung proposal of marriage and her own future.

I've enjoyed watching Chloe Grace-Moretz grow from role to role, and she is one of the most confident, self-possessed people I've ever interviewed. She is surrounded in her real life by family who are very protective of her and who also support her, and I think that solid foundation has served her well. She's often played preternaturally mature characters, but Annika isn't like that. She's just a teenage girl, and that's exactly what Megan likes about her. Things aren't complicated when she's spending time with Annika, and running away from her own life for a week gives her a chance to focus on someone else.

I was already enjoying the film, but when Annika's dad finally makes his entrance, things get kicked up to a different level, which is often the case when Sam Rockwell arrives in a film. He gives another great, unique comic performance here, and I love that I really couldn't predict where he was going from scene to scene. He and Knightley have instant and effortless chemistry, and Megan's week of solitude ends up complicating her life far more than she could have predicted. Kaitlyn Dever, so good in last year's "Short Term 12," also has a supporting role here as Misty, one of Annika's best friends, and she's a hilarious, crazy little animal in the role, the exact kind of teenager who will no doubt chew up and spit out my own boys in just a few years. Elle Kemper plays one of Megan's best friends, and she's great at playing a very tightly-wound control freak who doesn't understand any of what's happening to Megan.

"Laggies" has a terrible title that I still don't understand even after seeing the film, but it is charming and often seems to have some real insight to this arrested adolescence that seems to be more and more common, if not acceptable these days. Like all of Shelton's films, it's a little shaggy, but that's what I've come to like about her approach. She's happy to indulge small digressions, and she's not afraid to let characters act like people, complete with contradictions. A film like this isn't about the big sucker punch at the end or some complicated plot maneuver that ties everything together. It's a gentle, amiable, sincere little movie, and we could use about a hundred more Lynn Sheltons in this business, making movies that feel this lived in, this true. I'll be curious to see how they handle this one once they finally roll it out for a larger audience. It's worth your attention.

"Laggies" plays again four more times before the end of the festival, and if you're here, you really should check it out.