As the heroine's wacky, wine-swilling best friend -- named Missy, no less -- Marcia Gay Harden is much the worst part of ''p.s.," but she has a monologue two-thirds of the way in that shines a shaft of light on what's very good about the film. Missy and Louise (Laura Linney) are 40-something women besotted with a young man named F. Scott Feinstadt (Topher Grace), who shares the ridiculous name and sexy chutzpah of a dead boy from their youth. Talking with Scott, says Missy, makes her feel like her real self -- not the person she sees with a shock every time she looks in the mirror, not somebody's mother, but the girl imprisoned inside. And she's not about to let that go.
There are acrid, empathetic truths there, about the bipolar image adult women carry of themselves, about the way that moments of our youth grow to the totemic size of Easter Island statues as we age. As long as ''p.s." stays within that territory, it is smart, lusty, wistful, and acted with deep skill and heart -- exactly the qualities that made writer-director Dylan Kidd's 2002 debut ''Roger Dodger" such a razor-edged pleasure. Every time it strays outside, the movie falls on its face.
Louise is a rather less subtle version of the uptight-but-unraveling working woman Linney played in ''You Can Count on Me." She's director of admissions for the graduate art program at Columbia University, and we see her in the opening scenes rigorously applying her makeup and gazing with envy at students canoodling on the quad. Point taken: This corset is in dire need of loosening.
It gets unstrung with a resounding twang when F. Scott shows up for an admissions interview that tumbles helplessly into a seduction in the movie's first 20 minutes. Actually, ''seduction" implies patience; Louise is so in thrall to this throwback to her adolescence -- to the idea of youth itself -- that the sex is hurried, hot, disbelieving. ''Uh, is this going to hurt my chances?" asks F. Scott, not quite understanding what has just happened.
The film diddles around with the notion that the boy is Louise's lost love reincarnated, but it can't decide whether to run with that as metaphor or plot twist. True, that reflects Louise's overheated state, and both Linney and Grace -- who really comes into his own here, by the way -- do inventive things with the uncertainty. But the laser focus of ''Roger Dodger" is gone in this sophomore outing, replaced by more discursive, novelistic ramblings. Big surprise: Kidd wrote ''Roger" on his own, while ''p.s." has been co-adapted from Helen Schulman's 2002 novel by the director and the author.
Perhaps they should have heeded the writer's dictum ''kill your babies"-- said infants in this case being subsidiary characters such as Missy; Louise's easygoing wastrel of a kid brother Sammy (Paul Rudd); and -- red alert -- Louise's ex-husband Peter (Gabriel Byrne), a Columbia professor who volunteers a mid-movie confession that almost throws the whole shebang off the tracks. These represent paths Louise could choose, warnings she should take, and they clutter up ''p.s." like roadkill.
The central relationship is what matters, and Linney and Grace attend to it, she with a determination that flowers into greater awareness, he with the watchful comic aplomb of one of life's born improvisers. ''p.s." contains more than its share of implausibilities and absurdities -- and let's not even imagine the reception the movie would get if the genders were reversed -- but if it's not Linney's finest role, it contains some of her nerviest work. And if it's not Kidd's best film, that is surely yet to come.
Ty Burr can be reached at email@example.com.