"Guinevere," an affecting, gloriously acted account of a May-December romance, explores Woody Allen Syndrome from the sweet young thing's point of view. And this perceptive if contrived portrait also possesses the innocence, candor and abundant, often amusing detail of a teenager's diary.

"He was the worst man I ever met, or maybe the best," observes heroine Harper Sloane (Sarah Polley), who, at the ripe old age of 25, is looking back on her affair of five years earlier with fifty-something photographer Connie Fitzpatrick (Stephen Rea). "If you're supposed to learn from your mistakes, then he's the best mistake I ever made," she continues over the movie's opening montage of tasteful nudes photographed by her former lover.

Harper, all soft curves and adoring eyes, was 20 when she posed for the pictures, an act of both delayed rebellion and budding self-assurance. Ignored by her parents and upstaged by her older, prettier, smarter sister, Susan (Emily Procter), Harper was not only seduced by Connie but also rescued from a stifling future dictated by her bitter, domineering mother (the sensational Jean Smart).

The script by writer-turned-auteur Audrey Wells takes us back to Harper's first meeting with Connie at her sister's society wedding, a lavish affair followed by a champagne reception in the garden of the family mansion. Harper, shy and gawky in her bridesmaid's finery, slips away from the crowd with a magnum of champagne.

Connie, who is photographing the wedding, discovers her struggling with the unopened bottle and pops the cork with a little too much flair. He looks a bit dated with that white scarf tied raffishly round his neck, but Harper is intrigued by the charming Irishman. When she goes to pick up the wedding photos in his loft, she's wowed by his bohemian digs and retro-beatnik lifestyle.

Later she has a fight with her parents, and he invites her to move in with him--provided she work at becoming an artist and follow a serious study program that includes reading the works of Marx and Sartre. If she were a cockney guttersnipe, this might fly, but Harper is a college graduate who's been accepted by Harvard Law School. We accept that she's naive, but poorly read?

Unfortunately for Connie, Harper begins to wise up with a little sisterly advice from a friend and former lover of Connie's who confides that she's one in a line of "Guineveres"--the pet name he gives to all his mistresses. Each relationship usually lasts five years, the time it takes the student to outgrow the teacher.

Connie, portrayed with devastating honesty by Rea, truly loves his proteges but is incapable of maintaining a serious relationship with a mature woman. It's an obvious point that is underscored in a delicious confrontation between the downcast Connie and Harper's mother at her most castrating. "What do you have against women your own age?" she demands before launching into a raging condemnation of dirty old men immemorial.

Smart, at once sexy and incisive, raises the rafters with this man-eating moment, but Polley, a Canadian ingenue who was impressive in "The Sweet Hereafter," leaves little doubt: With this performance, a star is born.

Wells, a veteran screenwriter ("The Truth About Cats and Dogs," "George of the Jungle"), makes an assured debut behind the camera, with a great deal of help from cinematographer Charles Minsky ("Pretty Woman"). In the end, she fudges with a sentimental coda that reunites the Guineveres, all of them ever so much better for having known good old Connie. But then you're supposed to learn from your mistakes.

Guinevere (104 minutes at area theaters) is rated R for sexual content, brief nudity and profanity.

CAPTION: Sarah Polley, left, and Gina Gershon in "Guinevere," a tale of an affair from the ingenue's perspective.