Say When review: convincing, charming but a bit smooth around the edges

Lynn Shelton, the impressive auteur behind Humpday and Your Sister's Sister, graduates from the mumblecore circuit with this loveable comedy in which a loser in love meets an all-round, multi-purpose loser.

Megan (Keira Knightley) is what urban dictionaries call a laggy – indeed, Say When was released as Laggies across the Atlantic – an over-educated, underachieving drifter who just can't decide what to do with herself. A daddy's girl who's adoring father (Jeff Garlin) is happy for her to twirl a roadside sign while she figures out what branch of psychotherapy she wishes to specialise in, Megan feels increasingly alienated among her increasingly settled friends.

When her long-term boring boyfriend proposes at a Bridezilla chum’s wedding, our heroine panics and ends up spending the evening with a gang of skateboarding teens, led by cool kid Annika (Moretz).

And so it comes to pass that Megan embarks on a week-long sleepover at Annika’s place, so that she can get her thoughts together. But what will Annika’s uptight, pen-clicking dad (Rockwell) have to say about his daughter’s older chum?

Keira Knightley really does have some sizable acting chops. How else could one of the planet’s most sensible, together, Celia Johnson-voiced individuals convince us as a twentysomething, all-American slacker?

Happily, Sam Rockwell has thespian clout to match. Together their unlikely comic and romantic chemistry blazes up the screen. This is a sleeker film than Shelton's previous work, and it's impossible not to pine for the rough-hewn edges of her earlier, defiantly lo-fi work. But this is easily the most convincing, charming rom-com since, well, when did Your Sister's Sister come out?