There’s a simple, surprisingly affecting naturalness to Robert J. Siegel’s coming-of-age drama, which is also notable for showcasing the first lead film role of Lauren Ambrose (Claire Fisher, the rebellious high schooler of Alan Ball’s “Six Feet Under”). Ambrose gives a lovely, beautifully restrained performance as Frankie, the young waitress/half-owner of a struggling diner in a South Carolina beach town whose mundane summer existence is altered by the arrival of two strangers. One is Josee (Joelle Carter), the current girlfriend of a local lifeguard, whom Frankie’s brother Neil (Josh Pais) hires to wait tables for all the wrong reasons–she’s manifestly incompetent but sexy and approachable; one might expect her to become the villain of the piece, but she turns out to be, if calculating, also rather sweet, taking Frankie, who dresses like a tomboy and shows little concern for appearances, under her wing and encouraging her to come out of her shell. The other new arrival is Heath (Jamie Harrold), a scruffy hippie type who sells dyed T- shirts out of an old van and strikes up a haltingly romantic relationship with Frankie. The entrance of Josee and Heath into their lives threatens Frankie’s friendship with Nicola (Jennifer Dundas Lowe), a tattoo artist who’s hitherto been her almost inseparable pal.

What sets “Swimming” apart from most rite-of-passage movies is its gentle, unhurried pace, which allows the characters to grow and change like real people rather than a scriptwriter’s contrivances; they act in ways you wouldn’t predict according to the usual conventions of the genre, and the result is quietly convincing and touching. Siegel neither pushes too hard or belabors the obvious. He lets events unfold without undue exaggeration or clumsy melodramatics, and the result is a breath of fresh air–somewhat salty, given the setting. He also secures an absolutely luminous turn from Ambrose, who accomplishes more with the slightest gestures and most modest changes of expression than most young actresses do with all their labored histrionics. The performance is worthy of comparison with Ashley Judd’s star-making one in Victor Nunez’s wonderful “Ruby in Paradise,” which also dealt in a similarly understated fashion with a young woman forced to take hard decisions about her life. Carter and Lowe are nearly as fine; both are playing figures who could have easily degenerated into cliche, but they sidestep the pitfalls very skillfully. And Harrold is effortlessly charming as Frankie’s unlikely suitor.

“Swimming” is a small film, technically no more than merely adequate, but it tells its familiar story so unaffectedly that even the cliches seem refreshing; it never becomes glib or predictable, maintaining a thoughtful, realistic air throughout. The narrative it relates may not deviate greatly from other tales of a young woman’s liberation from a confining environment, but in this case it’s the grace notes that matter, and–thanks especially to Ambrose–they really sing.