''Swimming'' is a rarity among modern movies: a coming-of-age tale without cliche or sentimentality. Bolstered by a luminous lead performance from Lauren Ambrose, ''Swimming'' explores adolescent angst with refreshing honesty and empathy.

Set at a seaside resort in Myrtle Beach, S.C., the leisurely paced film uses its environment to convey a sense of transience: Relationships are fleeting here, and there's stuck-in-time malaise that overcomes the locals who eke out a living in the tourist trade, whether piercing body parts or serving meals in a steamy restaurant that touts its air conditioning more than its food.

At the center of the film is Frankie (Ambrose), a tomboyish teenager who, with her married, preoccupied older brother, runs the family restaurant that was willed to the siblings by deceased parents. ''Swimming'' conveys a quietly assured sense of Frankie's character from the moment she appears. Clad in baggy overalls and looking bored on a desolate beach while best friend Nicola (the moving Jennifer Dundas Lowe) lounges in a bathing suit, Frankie is tiptoeing into the waters of adulthood, eager but afraid to take the metaphorical swim.

Even the character's name conjures up associations: Red-headed Frankie - as played by Ambrose, the breakout star of HBO's ''Six Feet Under'' - is reminiscent of Julie Harris's definitive adolescent, also named Frankie, in ''The Member of the Wedding.'' Ambrose's Frankie is alternately plain and radiant, adventurous and cautious in this insightful, complex portrait of a girl on the brink of sexual maturity.

The catalyst for change comes in the swinging form of Josee (Joelle Carter), a pretty blonde who gets a job at the restaurant. The free-spirited Josee can't wait tables worth a lick, but her attractiveness and flirtatious manner guarantee that her ineptness will never be an obstacle in her life. Josee is the kind of young woman who is at ease with using her sexuality to get what she wants. Frankie isn't immune to her charms, and the younger girl becomes fascinated by Josee and her manipulative but flattering attention.

Director Robert J. Siegel handles this delicate material with grace and style. ''Swimming'' conveys the thrill and complexity of an adolescent crush, underscoring Frankie's nuanced character. Yearning for something more than the weekend partying on the boardwalk with piercing-shop proprietor Nicola, the repressiveness of the restaurant, and the condescension of her older brother, Frankie is eager to assume her own identity. The film nicely delivers two different characters into Frankie's world who help her figure out who she is and where her loyalties lie: the dubious but seductive Josee and Heath (Jamie Harrold), a likable drifter who sells T-shirts from his van.

Ambrose delivers an authentic, understated, beautifully etched performance that ranks with classics of the coming-of-age genre, which has been much maligned by Hollywood in recent years. ''Swimming'' is a finely crafted film that is all the more remarkable because it achieves its emotional power and moments of revelation with such delicacy, restraint, and ambiguity.

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